While we could argue about the state of Windows, Steam also did not have to engineer it this way and the requirement of launching through Steam feels deliberate.
From what I can find, as others have mentioned, the 8BitDo controllers don't require Steam to be running to work. I presume the PS5 controller likely also does not (I will test this later)
My PC may have installed something on its own, but I did not.
However I don't have an issue if there is a driver, the requirement of Steam running and apparently launching the game through steam is the issue here.
Edit:
I just realized I completely missed the not in your message:
> I can confirm that you do not need a Sony/8BitDo user account or any of their software running in the background for the controller to work.
so I guess my reply was not necessary, I need more coffee...
Valve does deserve criticism for the royal pain certain things are though. For example non-technical users will absolutely struggle to get Proton working without Steam, the process in doing so is purposefully kept undocumented and esoteric. There's 100% a little bit of undesirable obfuscation Valve does to push you towards just using Steam to run their OSS. It's definitely non-Free in the purist sense.