On reddit fitgirls site is never mentioned in full for good reason.

That’s just false. There’s a direct link in the r/piracy megathread.

That’s a problem solved by libreddit/redlib.

After googling, I think that was a joke about an unrelated app.

Lutris doesn’t, but you can do firejail --net=none lutris

OBS supports capturing audio from a specific application, so if you set that up you don’t need to be careful with foreign sounds.