I’ve seen Doom run on calculators, an Apple Watch, hell I think some guys at MIT got it running on an abacus.
There is an old meme about, “but will it run Crysis?” Turns out the Steam deck WILL run Crysis.
But not Doom (1993). Because that’s just too much to ask.
It doesn't specifically say it doesn't work, just that it isn't supported. Meaning you can probably get it to work, you're just on your own in doing so.
But it does run perfectly fine as-is. There's also much better way to run Doom with native GZDoom. There's also the Luxtorpeda utility that allows easy download and run for most (if not all) Doom ports for most (if not all) classic Doom games.
The Steam verification system is just bad and they mark a ton of games as unsupported even when they play nicely out-of-box.
Unless a game is known for having anti-cheat or some intrusive DRM/broken launcher, there's no reason to not try it (unless it's 100GB and you don't want to download something that won't run anyway)