TL;DR:

• Grab the OldUnreal latest version of UT99 from GitHub.
• Instead of a Steam, GOG, original disc, etc. copy, grab this zip file; it contains all the content without needing to extract it. The archive is known to work on macOS, and should let you skip most of the extraction steps on other platforms.
• Follow the instructions to copy it into the correct directory for your platform (application support on macOS, in the UT dir for Windows/Linux).
• Connect to ut99.scrollwheel.ca; password treehouse

btw here's comic chat 2.5