Ultimately I want to support devs who support Linux, but if it works on Proton doesn't it also get the flag on steam for support of SteamOS/Linux. I am only going by the Steam flag.
No, proton compatibility isn't really signified by the flag as it's not official support. That being said it seems Valve doesn't do much to help promote Proton compatibility either, as the only good resource I know to check compatibility is protondb.com (which you should take a look at by the way).
It doesn't get marked for every game that is playable. They have a list of games that are officially supported by Proton but there are so many more that work well that you can play if you enable the option in the Steam settings