Server side only anti-cheat is one of the problem domains that I'd really love to work on at some point in my career. This is the type of adversarial arms race that just seems really fun to think long and hard about.
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1. Determine minimum human reaction times and limit movement to within those parameters on the client side. (For example a human can't swing their view around [in a fps] in a microsecond so make that impossible on the client) this will require a lot of user testing to get right, get pro players and push their limits.
2. Build a 'unified field theory' for your game world that is aware of the client side constraints as well as limits on character movement, reload times, bullet velocities, etc. Run this [much smaller than the real game] simulation on server.
3. Ban any user who sends input that violates physics.
Now cheating has to at look like high level play instead of someone flying around spinbotting everyone from across the map. Players hopefully don't get as frustrated when playing against cheaters as they assume they are just great players. Great players should be competitive against cheaters as well.
No, those are still just as vehemently hated as “closet cheaters”, for example the whole XIM / Cronus infestation on any game that has controller AA.
It’s still possible to, on average, spot if it’s a closet cheater or an actual good player due to things like movement and gamesense, but for the average player it will be much less obvious, leading to a huge amount of rage towards good players because they are by default suspected as “just another closet cheater.”
A CS:GO player with good gamesense will habitually keep their crosshairs at head height and aim at corners where an enemy is likely to emerge. They'll have an intuitive sense of how long it takes to run from one point on the map to another. They'll listen through walls for footsteps to try and decode where the enemy are, where they're headed to and what strategy they might be about to attempt.
To the uninitiated, it looks a lot like cheating - you peek through a window and instantly get headshotted before you've had any chance to react. To the guy who hit you, it's just basic gamesense - you did a predictable thing and he punished you for it.