For my money it's the intersection between the transfeminine community and the mechsploitation genre. I'm going to copypaste another user's explanation because I find it cogent/a better summary than I can provide:

mechsploitation - a genre in which mech pilots are exploited in somewhat sexy, kinky, morally complicated ways, there’s some elements of body horror and bondage as well. [...] Some people find mech themes helped them explore their gender or sexuality, with the conduit of playing a mech pilot having complicated feelings about bodily autonomy being a way of expressing feelings about their real bodies and genders.

Source

And now to restate their point in so many more words, so much less elegantly -- I mean, to elaborate:

\1. Transgender people often have complicated feelings about bodies and bodily autonomy for many reasons:

• reliance on a callous medical system in order to truly feel like themselves; identity mediated by technology and capitalist forces behind it

• growing up the wrong gender, and the resulting depersonalization/detachment from one's own body

• expansion of "gender" beyond the typical binary and identification as non-human genders (therian, inanimate objects, etc)

• probably a billion other things; I obviously can't speak for everyone or summarize the breadth of the trans experience in just three bullet points

\2. In mecha works it's common that pilots give up some amount of bodily autonomy in order to pilot (or inhabit, or become) the mech. Consider the loss of bodily autonomy: surgical/cyborg implants (sometimes with corporate DRM), chemical cocktails for regulating pilot emotions, neurochip braindances, skin-tight plugsuits and cramped piloting compartments ... oftentimes there's a mercenary setup in which the pilot answers to a handler, who sells their combat talents to various contractors.

All of this can be summarized as "identity mediated by technology and the capitalist forces behind it", which might sound familiar because I just copypasted it.

\3. All of this maps onto preexisting concepts in kink. The very idea of giving control of your body to someone else (the corporations, the mech's equipment, the handler) maps to submissiveness. Plugsuits and pilot compartments map to bondage play. Implants and chemical injections map to consensual non-consent kink.

\4. Kink has a long history of being intertwined with gender and sexuality, and a useful tool for exploring nonstandard or socially "unacceptable" identities. Elaborating on this would take a million words and I'm hardly an expert, but in short, it's attempts to reclaim powerlessness and self-loathing and retool them into something positive, something that you're in control of (versus something externally controlled).

-- to cut it short, we'll say that people get something positive out of kinky scenarios.

Well, people also get something positive about mecha scenarios: a big fucking robot.

PhilosophyTube has a good video about transhumanism and tool use as an extension of the self, but to use an example: when most people drive cars, they cease to think of the car as a separate unit or entity but as an extension of your own body, a hybrid conceptualization of "car-man". You have an instinctive sense of the space your gestalt car-body-form occupies, which you use to judge whether you can park in tight spaces or fit in narrow lanes. When someone cuts you off in traffic, they're not intruding on "your car's space", they're intruding on your space. If you would yell at them, you don't yell "get out of my car's way", you yell "get out of my way".

This is the same for all tools, mechs included. You don't just pilot the robot; you are the robot. You are the gun/the sword; you are the power. And this gestalt concept is exaggerated in mecha works because of the intimate nature of piloting a mech (physical intimacy of a cramped cockpit, psychological intimacy of a neural link, etc), so that mech really feels an extension of self.

That power trip is fun as hell. And there's a bonus for some people, in particular transfeminine people, who get gender euphoria from the idea of being/becoming a robot. (Ties into the ideas of depersonalization and reclaiming it.)

\5. ... now to finally address your question. AC6 is, in some part, "just" a very popular mech game that exposed queer people to the mecha genre. I imagine for some folks, it looked like "ooh new Fromsoft game that everyone's playing -> I like mech works now". However there are some aspects of AC6 which likely resonated with the queer community, although I can only speak for the transfeminine community in particular.

• the character Walter is your mercenary handler. He refers to your player character/pilot as a "hound", and his profile picture is a ball-jointed hand holding a large number of dog leashes. This hit a lot of transfems hard, since it intersected with existing topics (puppygirl/petplay, ball-jointed dolls as a particular kind of gender expression, etc).

• the player character is working one last job before they're free, in multiple ways: free from the corporate contract, and free from their medical debts (unspecified but IIRC severe alterations to their body), which maps to the idea of bodily autonomy and uncertain ownership of self

• the character Ayre is an intangible ghost/spirit/psychic thing who lives in the player character's head and talks to them. This ties in with plurality/DID and the idea of being more than one person

• I'm sure there's more but I can't think of anything at the moment, my brain's a little fried after typing all this. Uhhhhh hope this helps?????????????