Presumably if there isn't a user report for whatever distro I'm using

Distro is unlikely to matter, as long as you don't pick a really obscure one. And even then, flatpaks will probably work fine.

Protondb also looks to be focused on Steam, I'm guessing it's like MacOS where if it's a game not on Steam then you're shit out of luck if there isn't a Linux specific version?

Steam is the easiest to work with and most well-supported. But there are other managers like Heroic Launcher and Lutris that can cover non-steam. Knowing whether a game can run on proton/wine outside of Steam does likely require a google search, though. Most things can be made to work as long as the game doesn't require kernel-level anticheat (e.g. Destiny and BF6).

I’m guessing it’s like MacOS where if it’s a game not on Steam then you’re shit out of luck if there isn’t a Linux specific version?

In addition to the options hamsterkill said, I run a lot of non-Steam games through Steam without issue. I've even run non-game software through it.

it’s like MacOS where if it’s a game not on Steam then you’re shit out of luck if there isn’t a Linux specific version?

No it's not. I don't even have Steam installed & play games just fine. You can use Bottles or Lutris to run the games, I recommend Bottles. The downside is that you'll have to learn how to use these tools. For example you'll not only need to fiddle with settings, but also with dependency dll files. Chatgpt can sometimes help & tell you which dlls are needed by which game. (at least when I asked it about Oblivion Remaster & Horizon Zero Dawn it knew)

Lutris has the advantage of community installers where you don't have to figure things out, but it's annoying that they'll force you to log into Gog Galaxy and such for your downloaded installers & Lutris may not have a force offline sandboxed mode if you don't want the software you run to report home.

If you decide to go with Bottles, one common rookie mistake is not giving permission to Bottles to the folders you install from and install to. You need to use Flatseal to do this nice and easy.

Heroic Games Launcher is another option, but it's less reliable than bottles, but it has a more noob friendly GUI & you can just directly log into an EPIC account & install away without having to use the epic launcher, which can be quite great if you have a big Epic library.

Here’s a nice list.

https://areweanticheatyet.com/?search=&sortOrder=desc&sortBy=status

Online multiplayer games are the most likely to have anti cheat. EA (Battlefield) is the most visible unsupported one. They view running under any virtualization or compatibility layer as an opportunity for cheating, so they intentionally deny it. EasyAntiCheat supports running in Linux, but not all game developers enable it. The success of the Steam Deck is starting apply pressure to change this, though.