There is plenty of consumer hardware that is supported on Linux, or will be as soon as a kernel developer gets their hands on it, reverse engineers the protocol if necessary, and adds support. For things like keyboards, there are often proprietary extensions (eg. for built-in displays, macros, etc.). It pays to check for Linux support before buying hardware though. Sometimes it's not the kernel drivers, but supporting software (eg. Steam input) that might not support it.

First class vendor support for Linux is more common for niche/premium hardware designed in the west, than cheap chinese knockoffs that follow it. Long term customer support is not their strong suit.

What do you mean lacking support for keyboards and controllers? Maybe for doing weird custom stuff like RGB, but for anything else they're standard HIDs and will work with anything, no "support" needed. You can plug a USB keyboard and mouse into your phone and it'll work if you want.

I'm currently playing Clair Obscur on linux through steam with a cheap fake xbox controller I got off ebay, and it works perfectly. I'm using an Nvidia card too, and I haven't had to do any customisation or anything.

Easy anti-cheat won't work, so Valorant/Fortnite, etc. are out of the question for now, but any games that don't use that kind of malware are probably fine.

Likewise. Don't expect it from China though.