https://linuxgsm.com/servers/

In the style of those games, self hosting a Vintage Story server is extremely easy to do. Extremely easy to customize, well documented wiki.

I've been running mine for over a year with zero hassle on a tiny server.

I've been thinking about hosting a Quake II server, lately.

EDIT: Or Unreal Tournament. Both were my jam back in the day.

Could try hosting a Multi-User Dungeon (MUD). I tried to host an Evennia server last year but could never get it to work properly. Wasn't a big deal though cause it's not like I had any friends who wanted to play, lol.

Space Engineers?

I set up roller coaster tycoon 2 using the OpenRCT2 docker container , pretty cool.

I ran a fairly popular RTCW server back in the day... Insta-gib and sniper rifles only. Good times.

UT2004

Final Fantasy XI is still a beautiful game and is self hostable, but as with most MMORPGs, you need lots of people to get the full experience. Luckily if you're hosting your own, you can always mod settings to make yourself lvl 99, turn off enemy AI, teleport anywhere, or whatever.

Starseige tribes? Trackmania?

We've spent last few months playing Enshrouded. The dedicated server is a resource hog tho, even on an empty server it'd use a ton of CPU time. Still lots of fun with a few friends exploring and building stuff

To add my own two cents into this mix.

RuneScape classic with the fan made OepnRSC

And Final Fantasy XI ... kiinnddaa. Not sure if that one's feature complete yet or ever will be. But there's a fan server effort for it like RuneScape.

Mindustry

Project zomboid

I've had a blast with AssaultCube couple of years back.

Spun up a server, played with some friends, then random people started showing up and we played with full server for hours.

It was so much fun, I hope I find free time to do that again.

Sonic Robo Blast 2 Kart or Doctor Robotniks Ring Racers. It's a good time if you have friends and there are still regular players online.

Valhiem

My two most popular public game servers are Necesse and Rimworld Together. Neither have many public servers.

https://github.com/RimWorld-Together/Rimworld-Together

https://store.steampowered.com/app/1169040/Necesse/

I'm amenable to luanti with voxelibre.

Though I don't like the rebranding. Luanti sounds like a product you'd hear about in a drug commercial. "Side effects of luanti may include chronic flatulence, erectile dysfunction, and death"

Heroes of Umbra (maple-story-like)

Veloren

I ran a 7 days to die server for a few years, fun to do missions with a lot of friends, and build bases and stuff.

Two that I run for our little group outside the ones you mention are Space Engineers and Valheim

Edit: Space Engineers is a little annoying though, you either have to use some emulated / translated setup, (I think I saw some being cobbled together by others), or you have to run a Windows Server VM.

Armagetron ?

Still going...

I must dig out my copy of Doom3 and see if those servers are still going...

Vintage Story with mods. Vintage Story is a minecraft-like wilderness survivor game focusing on long term survival, not just "eat berries". it's under active development. https://mods.vintagestory.at/vsds the server docker image if interested.

V-rising

A sort of diablo-style game where you are the bad guy vampire. Very fun, but the damn server doesn't pause time when it is empty, so I can only spin it up when I plan to play, not have it on all the time like valheim if my friends want to jump in without me and play

Not quite a server, but Total Annihilation. Awesome times. Not sure if supreme commander does it too.

Project Zomboid is a blast, especially when you really dig into the options for changing game rules. You can basically craft your own custom zombie apocalypse. You can decide how the virus works, whether zombies are slow or fast, whether they have good eyesight, good hearing, how strong they are, where they spawn. You can change loot rarities, how long it's been since the outbreak started, when the power gets shut off, etc, etc.