Its all in the title.
I know our UT2k4 server hasn't seen a lot of activity lately but this is still sad news.

https://www.epicgames.com/site/en-US/news/epic-is-turning-off-online-services-and-servers-for-some-older-games

Oh no! Does that actually matter to us, though?

It looks like this only means the master server is going away, which means you won't be able to browse servers in the game without configuring it to use a community-run master server- however Epic's messaging on this is too vague to be certain that it's the case (perhaps there's some other Epic service that the game depends on under the hood?).

Somebody seems to conclude that the games will still work in the way I've guessed:

However I know UT2004 requires some kind of CD key validation for online play (and guess that UT99 and older ones don't) so it may be that the CD key validation will break?

From what I recall our server already has CD Key validation turned off. It was already optional since at least in the early days most of our members copies were less than legit. But that might be something we have to look into.

Since we aren't on any of the community master server setups we don't really have to worry, the main reason private servers would want CD key validation turned off is to ban cheaters by their CD key but since our server isn't discoverable that is less of an issue.

The biggest problem with the server shutoff is that new players aren't going to be able to find servers unless they figure out that they need to edit their INI file to add a community master server, they make such change correctly, and the community all agrees to use a single community master server (or community master servers communicate what game servers they are aware of). UT2004 already allows custom master servers, the protocols are apparently understood, and people are working on building community master servers. The game works fine unplugged from the Internet, and if it didn't, someone would patch the game to fix that pretty quick anyways.

Third-party anti-cheat (Llama Punter, which is apparently working on the loss-of-master-server issue as well) works a lot better than inbuilt CD key validation, since cheaters are smart enough to use legitimate/shared/stolen CD keys nowadays. Llama Punter unfortunately (and necessarily) bans some common CD keys, such as GOG's default CD key, but they can be manually exempted by GUID.