

Because Linux doesn’t have any legitimate programs to enforce the DRM on their files, presumably. Plus their .exes/.dmgs for companion tooling won’t work on Linux, unless you’re the kind of person smart enough to bypas this message.
To be honest, the code generating this message probably just checks if you’re on at least Windows 10 or macOS 14 to make sure their code works, and just echoes back the OS if it’s not supported (Android, iOS). I doubt anyone bothered testing for Linux on their website, let alone care to fix the problem.
I can’t say I care too much about this feature, but looking into it I can’t seem to find any documentation about it at all. Where are you getting this confirmation that you can’t disable it?
I’d just stop using Steam Chat if it’s not working for you, there are dozens of game chat services specifically built for chatting in games. Then again, I’d probably stop playing with people that make me self–conscious of typing indicators as well.
Also lol @ anything Valve designs being “modern”. Steam Chat looks, feels, and behaves like MSN Messenger did twenty years ago. Don’t let the gradients fool you.