There really should be another level to it:

Tinker: runs fine with some in-game config or with Steam OS tools like Steam Input

It would exclude any lower level tweaks like changing launch args or using a special Proton version, those can stay unsupported. Basically, if you can get it working well intuitively without looking stuff up online, it should have some level of support.

I've played several "unsupported" games that work fine, so something should be done here.

But that’s just it. That fiddling isn’t something the dev probably accounted for, so it should be noted that the experience will be less than optimal. I think it still fits.

Are using Decky loader? The ProtonDB badges plugin is great for this!

Battlefront 2 - unsupported officially, but actually plays fine with no tweaking and gets a gold badge on protonDB.

Having the plugin shows the good badge on the games store and installed page

It's more of a status for publishers and Devs to hit.

I’m pretty sure people have been playing Tales of Berseria on Steam Decks for 3 years and it still says unsupported. Seems perfect to me