Trying to get a list of older games that have a lasting player-base.

I'll give a few examples:
Mount and Blade: Warband
Battlefront 2 (2005)
Doom has a few online clients with consistent players
Morrowind (TESMP) is getting more popular
Urban Terror (free ureal3(?) shooter) always people playing

Trying to avoid steam games, but I can think of a few source mods and HL1 engine games that still maintain a healthy/sustained community.

What other games do you think fits this vibe?

hmmm
how can you pay morrowind online? is there a mp part now?

https://tes3mp.com/

It's an add-on for the OpenMW project.
https://openmw.org/en/

Maybe Terraria.

World of Warcraft
Tibia
Runescape
Ultima Online
Diablo 2

thx
is this good
I cant see how a single player designed rpg game can be good in multi

Neverwinter Nights

With friends, yes. Not sure about wider community.

If you consider a game from 2009 old, then Minecraft. It still receives updates to this day, too. Sadly no proper DRM free release that I know of.

Final Fantasy XI also has servers up and running since it was released back in 2002. Just not sure if it still has a strong player base.

I haven't checked recently but not too long ago there was still a very healthy online community for Heroes of Might and Magic III