I think Titanfall 2 is still on sale on steam for uh... 5 dollars.

Its got Northstar, a custom client that allows for private multiplayer servers... also works on linux, literally has its own custom proton version.

Oh and there are mods as well, guided installers, mod managers, etc, for windows and linux.

Runs great on a steam deck!

... and looks ... basically the same as a shooter from 10 years later, at least at 1280 x 800?

(its built on a custom forked version of the portal 2 source engine, so it actually runs efficiently and looks good =D)

Doesn't have a huge playerbase, but it is decent enough that you can probably find a few well populated servers, at least in NA region.

... looks like titanfall 3 got turned into an extraction shooter and then cancelled.

So anyway yeah, hilariously its time to return to tradition for enthusiasts of many old school competetive games from before the bullshit of endless battlepasses and MTX kicked into high gear... and as others have pointed out, the indie scene is full of gems.