Somehow Xonotic manages to be both completely free/open software and not have cheating problems like this. It's never been clear to me how they've done that although client-side stuff like these kernel anti-cheat things were obviously never going to work.

Combination of niche/low user base, community servers encouraging user-based enforcement of norms, and the lack of a unified ranking system. People don't cheat if it doesn't psychologically reward them. (at least en masse)