Xonotic is from a different reality were competitive movement shooters never died! It is the active fork of what used to be called Nexuiz.
Xonotic runs on the darkplaces engine, it isn’t just stylistically of the arena shooter era it is directly descended from quake and the lot.
Playerbase is small but consistent, Xonotic is very easy to download game (it is on several linux repos for one) so there are always new people wandering in and a solid community of veterans.
High level competitive gameplay is absolutely wild!
Xonotic “defrag” is a community of people who just strafe jump race, Xonotic is very popular for that given the buttery dream-like highspeed movement.
(beginner tip the blaster is your best friend, you always spawn with it and it massively elevates movement in Xonotic)