The original CDs of UT 99 had the linux Binaries on them, but (as mentioned in the UT2004 comment) Atari Castrated the Steam version.

Use https://flathub.org/apps/com.epicgames.ut99 to play UT 99

it used to have one though; don’t just look at the Steam Page :)

You are missing that Atari castrated the Steam version since Steam is only windows (at that time) they thought they wouldn’t need to ship the linux binaries with the downloadable version of UT2k4.

So you get the following options:

• Use the UT2k4 linux setup from the original dvd
• run UT2k4 via wine/proton
• use the Flatpak launcher, copy the gog/steam data into the appropriate folder (the flatpak runner will tell you where)

via the latter I got this:

Since release.