A libre Unreal Engine 1 when? The first Deus Ex it's better. Doom I-III, Quake I-III engines are under a libre license. UE1 was always missing there.
I would love this. UT99 has aged wonderfully (gameplay-wise, anyway) to this day, but getting it running on modern systems has always been a pain. Ironically, I've had better luck getting UT99 running under WINE in Linux than on Windows 7+.
It should work well on all platforms considering it's still being patched https://github.com/OldUnreal/UnrealTournamentPatches