Server side is beatable as in, you could inflate your skill to that of a professional player.

The optimal serverside anti cheat would be able to recognize what gameplay is human level, and what gameplay is impossible or very unlikely to be human, and make punishment decisions based on that.

Then, the best cheat would just be almost perfectly simulating a pro player, and at that point the cat and mouse game of anti cheat and cheating would be far far less relevant.

Something like blatant tf2 spinbotting, or scoping someones head through a wall right before peeking them in r6, are absolutely detectable serverside with heuristics or machine learning models or etc, and that should be worked on rather than embedding some spyware into my uefi firmware or whatever.