BTW, this is the actual oldest server (and it's still up): https://minecraftonline.com/

This is a close contender (it's is also up, and really thriving; it's even on close to the latest version): https://en.wikipedia.org/wiki/2b2t

I can recommend 2b2t if you don't mind the 4chan-like mentality in the chat. Probably the only server where coding is actually a desired, core skill in factions/teams/groups.