I wish UT99 was still active. I retired from it years ago
It still has some activity for CTF. A number of modded servers (low gravity Instagib, sniper) are around. There are also a few regular weapons CTF servers that are still populated in addition to 5v5 clan match-style PUG matches that are organized over IRC.
Here are some server links to regular weapons CTF servers:
High Speed CTF (all CTF-Face) server
Terror Squad Clan all CTF-Face server
USonic www.UnrealArgentina.com server (Apparently there's an active UT99 fanbase in South America. They even have clans and supposedly there is clan activity.)
Xoldog 400 server (This is a German server that runs standard regular weapons CTF and custom maps, but with a ghey translocator throw limit mod. It's often populated during European prime time.)
PUG Matches: You might also enjoy 5v5 clan-match style PUG matches where people use Teamspeak for voice comm. In a PUG, two captains draft-pick the other 8 players for their team with the idea being to get balanced teams and a more intense game. They are organized on IRC (Internet Relay Chat) using a pugbot program. You can find them here:
Server: irc.GlobalGamers.net
Channels: #latinopug
#mlut (North American pro community)
#speedpug