PART I: CONTEXT
A woman must not put on man’s apparel, nor shall a man wear woman’s clothing; for whoever does these things is abhorrent to your God ה'.
At first glance, how does this verse apply to trans people? It has certainly been used to deny trans Jews access to a HaShem-ordained transition (see here). However, if we say that a trans woman is a woman, and a trans man is a man, then this verse seems to apply to them in the same way that it would apply to any other woman or man. As Alexandra Kohanski explains in Be Whole: A Halakhic Approach to Gender & Transition, trans people must be trusted to "know their own gender, and that a person’s knowledge about their own gender carries nearly total power to override any contradictory claims."
That seems to wrap this up without even checking out the Talmud - but let's dive deeper. After discussing restrictions on shaving for Nazirites, the Mishnah says this about 22:5 -
Rather, it means that a man may not wear a woman’s garment and thereby go and sit among the women; and a woman may not wear a man’s garment and sit among the men.
So, a "man" cannot wear a "woman's" garment and sit among "women". This seems to point to the idea of deception, of men trying to blend in with women.
Rashi adds a bit more in his commentary:
THE APPAREL OF A MAN SHALL NOT BE ON A WOMAN — so that she look like a man, in order to consort with men, for this can only be for the purpose of adultery (unchastity) (cf. Sifrei Devarim 226:1; Nazir 59a).
NEITHER SHALL A MAN PUT ON A WOMAN’S GARMENT in order to go and stay unnoticed amongst women. Another explanation of the second part of the text is: it implies that a man should not remove the hair of the genitals and the hair beneath the arm-pit (Nazir 59a).
FOR [ALL THAT DO SO ARE] AN ABOMINATION [UNTO THE LORD THY GOD] — This implies that the Torah forbids only the wearing of a garb that leads to abomination (unchastity) (cf. Sifrei Devarim 226:1).
Again, this does seem to be about things that lead to "abomination" (תּוֹעֵבַה) through deceit.
[See also Siftei Chakhamin, Deuteronomy 22:5:1-2 and Ibn Ezra on this verse]
1. ritual sense: ... b. to God and his people: ת׳ of unclean food Dt 14:3; worshipper of idols Is 41:24, cf.Je 2:7; various objectionable acts: ת׳ לפני י׳ Dt 24:4; ת׳ י׳ 7:25 (חֵרֶם, cf. v 26), 17:1; 18:12; 22:5; 23:19; 27:15; offering of children (עשׂה) 12:31, cf. Je 32:35, also pl. 2 K 16:3 = 2 Ch 28:3, Dt 18:9, 12 (+ witchcraft); idolatrous practices (sometimes with other illegal acts)
BDB defines this usage of the word as "various objectionable acts", and a half (4 out of 8) are about idolatry. Most of the rest are about sexual perversion. Rambam points to this combination directly in Guide for the Perplexed:
The same is also the reason of the precept, “The woman shall not wear that which pertaineth unto a man” (Deut. 22:5). You find it in the book Tomtom, that a male person should wear coloured woman’s dress when he stands before Venus, and a female, when standing before Mars, should wear a buckler and other armour. I think that this precept has also another reason; namely, that the interchange of dress creates lust and leads to immorality.
[See also: Mishneh Torah, Negative Mitzvot 40, and Part 3 37:5 of this same text]
PART II: WHAT NOW?
However, in a world where Venus-worshiping crossdressing orgies aren't all that common, how can this mitzvah be brought into our world today?
It doesn't seem to change anything about the fact that we know that trans women are women and trans men are men. If a trans woman is wearing a woman's garment, that's hers to wear as a woman. Additionally, she can even wear pants and other traditionally masculine clothing:
A woman should not adorn herself as a man does - e.g., she may not place a turban or a hat on her head or wear armor or the like. She may not cut [the hair of] her head as men do.
A man should not adorn himself as a woman does - e.g., he should not wear colored garments or golden bracelets in a place where such garments and such bracelets are worn only by women. Everything follows local custom.
[See also: Sefer HaChinukh 542:4 and Shulchan Arukh, Yoreh De'ah 182:5]
I'd say it's fair to say that local custom within my community is that women can wear some traditionally masc clothing (like pants). A modern reading of Rambam might say that if you get it in your local boutique's women's section, it's ok for a woman to wear (other tzniut-related discussions aside).
However, how about non-binary people? So far, this reading centers binary trans identity. Rambam can also help us out here. A few sentences after the previous quotation, he says this (italics indicate footnotes):
(י) לֹא תַּעֲדֶה אִשָּׁה עֲדִי הָאִישׁ כְּגוֹן שֶׁתָּשִׂים בְּרֹאשָׁהּ מִצְנֶפֶת אוֹ כּוֹבַע אוֹ תִּלְבַּשׁ שִׁרְיוֹן וְכַיּוֹצֵא בּוֹ אוֹ שֶׁתְּגַלֵּחַ רֹאשָׁהּ כְּאִישׁ. וְלֹא יַעֲדֶה אִישׁ עֲדִי אִשָּׁה כְּגוֹן שֶׁיִּלְבַּשׁ בִּגְדֵי צִבְעוֹנִין וַחֲלִי זָהָב בְּמָקוֹם שֶׁאֵין לוֹבְשִׁין אוֹתָן הַכֵּלִים וְאֵין מְשִׂימִים אוֹתוֹ הַחֲלִי אֶלָּא נָשִׁים הַכּל כְּמִנְהַג הַמְּדִינָה. אִישׁ שֶׁעָדָה עֲדִי אִשָּׁה וְאִשָּׁה שֶׁעָדְתָה עֲדִי אִישׁ לוֹקִין. הַמְלַקֵּט שְׂעָרוֹת לְבָנוֹת מִתּוֹךְ הַשְּׁחוֹרוֹת מֵרֹאשׁוֹ אוֹ מִזְּקָנוֹ מִשֶּׁיְּלַקֵּט שַׂעֲרָה אַחַת לוֹקֶה מִפְּנֵי שֶׁעָדָה עֲדִי אִשָּׁה. וְכֵן אִם צָבַע שְׂעָרוֹ שָׁחוֹר מִשֶּׁיִּצְבַּע שֵׂעָר לְבָנָה אַחַת לוֹקֶה. טֻמְטוּם וְאַנְדְּרוֹגִינוּס אֵינוֹ עוֹטֵף כְּאִשָּׁה וְלֹא מְגַלֵּחַ רֹאשׁוֹ כְּאִישׁ וְאִם עָשָׂה כֵּן אֵינוֹ לוֹקֶה:
A tumtum and an androgynous [whose status with regard to gender is doubtful, as explained in Halachah 4] may not wrap their heads [in a veil] as women do, or cut [the hair of] their head as men do. [As the Rambam states in that halachah, "the stringencies of both a man and a woman are applied to them." Hence, they are not allowed to clothe themselves in a manner which is distinct to either a man or a woman...] If they do [either of the above], they are not [liable for] lashes. [because we are unsure of their gender. Accordingly, it cannot be definitely said that a prohibition has been violated.]
I read "tumtum" as being able to encompass modern non-binary identities. The definition for tumtum that we have in the Talmud emphasizes the uncertainty of their identification/sex (then thought of as indistinct). Non-binary people exist in this same inbetween or indeterminate space in their gender identification. And, to once again rely on Kohanski, we take them at their word, no matter their genitals.
Non-binary people, as the sacred beings that they are, remain beyond gender. Although they are obligated in distinct ways to perform certain mitzvot, they are not liable for how they might choose to follow them.
Therefore, a trans-centered reading of Devarim 22:5 could be that trans people aren't trying to deceive anyone with their gender performance and non-binary people are allowed to do as they like; after all, we hold that a trans person does "know their own gender, and that a person’s knowledge about their own gender carries nearly total power to override any contradictory claims." This interpretation brings us right back to where we started, while also allowing me, as a woman, to wear (feminine) pants.
This leads us to a potential modern reading of this negative mitzvah:
People are not allowed to wear clothes that match any other gender identity other than their own, as this allows them to deceitfully hide their gender in order to commit abominations.
PART III: TRANS THEORY ADDS MORE
However, I think we can do more with this mitzvah using critical trans theory. As the (Jewish) gender theorist Judith Butler famously wrote in the '90s:
Performative Acts and Gender Constitution, 528
Gender cannot be understood as a role which either expresses or disguises an interior 'self,' whether that 'self' is conceived as sexed or not. As performance which is performative, gender is an 'act,' broadly construed, which constructs the social fiction of its own psychological interiority ... Genders, then, can be neither true nor false, neither real nor apparent. And yet, one is compelled to live in a world in which genders constitute univocal signifiers, in which gender is stabilized, polarized, rendered discrete and intractable. In effect, gender is made to comply with a model of truth and falsity which not only contradicts its own performative fluidity, but serves a social policy of gender regulation and control. Performing one's gender wrong initiates a set of punishments both obvious and indirect, and performing it well provides the reassurance that there is an essentialism of gender identity after all.
For me, this destabilizes the interpretation of Devarim 22:5 as a prohibition on deceitful gender performance. Like Butler, my gender performance is illusion - it code-switches from place to place and person to person. I do gender for my own personal gain and safety. All people do this too - whether you are cisgender or trans. This verse seems to also ban binary-gendered people from doing any form of drag or gender play.
So, what do we do with Devarim 22:5? For a possible answer, the kabbalist rabbi Isaiah HaLevi Horovitz's book of commentary has this to say -
The basic function of garments [prior to Adam's sin and the need to use garments as protective covers. Ed.] is to portray the glory of the wearer. This is why Adam, while in גן עדן, wore garments constructed of light, i.e. he radiated light, showing all those whom he approached that he was a creature of substance. Once these garments were exchanged for כתנות עור, tunics made of skin, their function became reversed. Clothing now was designed to hide the shame of the wearer.
While still in Gan Eden, Adam and Eve are clothed in light. Their presentation isn't particularly gendered, they just are. After eating the fruit of the tree of knowledge, clothing became about shame, about hiding that inner light. Clothing, shame, and gender become a curse.
As the Kabbalists taught, following mitzvot brings a person closer to that divine state in Eden. When you take Devarim 22:5 out of the context of enforcing the hierarchical gender roles that come after Eden and center it on that attitude of bringing oneself closer to Eden, it seems to be saying that it is a mitzvah to not betray (or deceive) yourself in your gender performance. To interpret it as pushing for visible divisions between men and women brings that person farther from Eden. Hierarchical gender roles bring us farther away from both Luria and Kaplan's concepts of tikkun olam.
Outside of pikuach nefesh and the obligation to preserve one's own safety (Devarim 4:15), 22:5 requires a gender performance that is affirming and supportive of how each individual person wants to play with gender, regardless of their gender identity.
Everyone has their own idea of who they are in relation to the gender binary. For some, that comes with a sense of gender playfulness. All performances of gender that are true to an individual are not forbidden by Devarim 22:5; on the contrary, it is forbidden to pursue any performance that causes gender discongruity, or dysphoria.
Therefore, I do not read Devarim 22:5 to be an obligation to not deceive others with your gender performance. Instead, a trans-centered reading of Devarim 22:5 forbids gender presentations that are deceitful to oneself.
This provides another example of how gender transition is a mitzvah for trans Jews, not something unrelated to their Jewishness, and definitely not something that tarnishes it. A Jewish transition points back to the divine state of being in Gan Eden - something truly beautiful.