Unreal Tournament patches are still in development with official approval/support of Epic:
https://github.com/OldUnreal/UnrealTournamentPatches/
These patches include updates to the master server addresses which enable the game to continue working online.
Third-party master servers have also been available for at least a decade.
UT99 is still being maintained by the community if you want to give it a try!
You can find the latest patches on GitHub.
It should work well on all platforms considering it's still being patched https://github.com/OldUnreal/UnrealTournamentPatches
Speaking of UT99... it's still getting patches https://github.com/OldUnreal/UnrealTournamentPatches
> OldUnreal took over maintenance of the Unreal Tournament code base after reaching an agreement with Epic Games in 2019.