Battlefront 2 (the original), still active when the servers have been down for years
Titanfall 2. Official servers aren’t technically down, but pretty much unusable and NorthStar is the alternative
Counter strike 1.6 is pretty much just community-run servers, same with day of defeat: source. I don’t know if they are tied with valve that if valve shut them down, they wouldn’t be searchable.
Supreme commander: Forged Alliance
Hell, Battle for Middle Earth II still has a small community
Valheim has never had official servers. I run my own via docker on debian
Unreal Tournament 1999
Minecraft (official servers aren’t down, but if they shutdown there would still be 2000 servers)