Open-source one: Xonotic

https://xonotic.org/

This is the one that I still fire up with my friends once in awhile. The only problem is it sort of lacks the charm and nostalgia, and it's a bit rough around the edges in parts (the gun selection is fairly abysmal and don't feel right IMO).

Outing myself as a parent of young kids here, but I have been genuinely surprised how good Hypershot on Roblox is. It reminds me of Unreal Tournament a lot and is really easy to play, I've been hooked for a few weeks. It cuts everything down to the basics like UT without all the bells and whistles modern AAA games have. Oh and it's totally free (but with all the monetization attempts Roblox games tend to have, though they aren't necessary to play even at a high level).

Diabotical was heavily inspired by Quakeworld/Quake3/UT arena shooter FPS.

https://www.diabotical.com

https://www.warsow.net/ is good and runs on pretty much everything. Plays like a mix between UT and Quake3.

I love Warsow but am I right that it's very hard to find an opponent these days? I just checked https://arena.sh/wa/ and there are 4 non-empty servers, but most (maybe all) of the players seem to be bots.

I haven't played a while so I cannot comment. When I last played I spun up a server for my friend group to play on. This is the beauty of old school games like these, no need to rely on a company to keep servers running. In a way Warsow is the perfect LAN game: everyone can run it and it's easy to host a server.

there are but they are pretty niche, so mostly you play against bots or against friends.

This one is the last one I heard of but I also haven't followed the scene much lately: https://store.steampowered.com/app/324810/TOXIKK/

TOXIKK was awesome. But very hard to find matches with players.

Not that I can really think of - there's the adjacent Sauerbraten, but that's a similar vintage!

If you like 1v1/2v2 gamemodes, Straftat is pretty amazing: https://store.steampowered.com/app/2386720/STRAFTAT/

Arena shooters have been relegated to side modes in other, bigger games. Probably the last big shooter with that as its focus was Halo: Infinite, but that struggled to stay relevant and its next big update will be its last.

Splitgate captures the feel nicely. Simple, fast, and last I checked free. Very recommended.

maybe Splitgate if that's still alive?

Splitgate 2 is scheduled to launch later this month.

Tribes 3: Rivals is a load of fun. Flying around on jetpacks helps raise the skill ceiling.

If you want something wild and frenetic check out Chivalry 2. It's an FPSlasher

Please point me to the nearest meat grinder. I'm jonesing for breakneck speed mayhem!

Isn't that what Overwatch/Valorant/Apex/Fortnite etc are?

Overwatch is a objective-based game, and like Valorant is a "hero shooter". Apex and Fortnite are battle royales.

I think the closest I got was The Finals but still class-based, so reminds me more of Team Fortress.

I loved playing 1v1 on Quake 2/3 and UT, also team deathmatch, from the list you commented it feels like each game got one of those aspects but none that makes the genre of UT what it is: knowing where weapons/ammo/armor spawn, map knowledge to navigate around, emergent movement mechanics (rocket jumps, strafe-jumping, etc.).

Interesting to see this genre mostly died out, and remnants of it have been scattered across other genres.

No, these are class based team FPS, core of UT and Quake is (team) deathmatch

The Unreal Tournament series are arena shooters[1] which has sadly died a death, partially due to Epic Games negligence.

[1] https://en.wikipedia.org/wiki/Arena_shooter

Shootmania was supposed to be it.

Dusk is more Quakeish than UTish, but well worth a look. The graphics are deliberately low fidelity 90s retro, but the gameplay is tight.

Not really unfortunately. This video goes a bit into why (https://www.youtube.com/watch?v=F-t37idOfvk), but the short version is the skill floor (or ceiling?) was too high, making it unwelcoming for people to just pick up, unlike a COD-like shooter.

Halo 3 multiplayer had this solved by maintaining a dynamic rating of the players so that it could match skill levels when random matches were made.

Quake Champions?