Sadly Epic have announced they are turning off online services and servers for some older games, this includes UT 99, UT 2003 and UT 2004. For the Unreal Tournament (UT) games this is essentially the master server that lists all the available servers you can join from within the games menu.
As PC gaming was pretty open back then you can still join local network games or internet games via a direct IP connection, regardless of if Epic's servers are online or not.
Digiex has kept the Digiex UT 2004 Server online for pretty much the last 15 years now (how time flies!) which you can join via a direct IP connection.
However, if you wish to easily find other servers to play in then all is not lost, their is a community run Master Server for Unreal Tournament 2004 by OpenSpy that you can easily switch over to using, this will allow the in game server browser to display any server which has also moved over to using OpenSpy.
To switch over to OpenSpy all you have to do is edit the master server address in the UT2004.ini file, I've posted a mini tutorial showing how to do this: Unreal Tournament 2004 Internet Play Fix - How To Update The Master Server.
So anyone who wants to can keep enjoying UT online play for years to come. The internet might disagree, however I still think Unreal Tournament is better than Fortnite!