Not the parent, but I still regularly (let's say once a month) Wolfenstein: Enemy Territory (2003) [1] with a number of friends, although it has now a community that keeps developing/maintaining the open source code [2], so I'm not sure it if counts as dated
Yep! I play on the FA and Team Muppet servers, and at peak times during the day the server will be almost full (60 players).
But, that's kinda it, I doubt there are ever more than 1k people online at a time. But those same people have millions of XP (I actually have a PR open on the client ATM to abbreviate the XP in the scoreboard, which I think we finally need after 20 years lol)
If you play, make sure you use the etlegacy client, it is backwards compatible with many fixes/improvements.
[1] https://www.splashdamage.com/games/wolfenstein-enemy-territo... [2] https://www.etlegacy.com/