Steam / Valve doesn't care if you pirate games as long as you don't involve their servers. This is a pretty known / accepted fact.
If you really are worried just install a different OS on steam deck and don't use steam at all.
I don't think I would personally really consider using windows for anything these days. Semi-ironic to be concerned about steamOS spyware when you are comparing to windows which for sure has spyware.
Do you have a source on this? Everything I have read from them so far about the news today makes this sound like a temporary problem that was out of their control.
https://itch.io/updates/update-on-nsfw-content
I'm pretty confident they will get this sorted out. It doesn't seem like they have any intention of banning nsfw content.
Edit: I'm responding here about NSFW content because I was confused. I don't think Itch actually ever banned queer/LGBTQ content in any way. The recent drama has been over NSFW content specifically so I assumed OP was specifically taking about NSFW queer content.