Everyone can squat. Crazy to say otherwise.
Limited dorsi-flexion
Limited thoracic mobility
Tight hamstrings
High hip structure/short torso
Limited external rotation
Abnornal anterior tilt
Abnormal kyphosis
Abnormal lordosis
Both kyphosis and lordosis
Abnormal scoliosis
If they can't perform an overhead squat meeting certain requirements then I don't squat them. By squat I mean a traditional squat. That doesn't mean after weeks or sometimes months of soft-tissue to get them mobile I won't attempt a goblet, front or box squat (in some cases). But if they're structure doesn't allow it, we don't do it.
I rarely if at all squat basketball players over 6'4 unless they can pass an OH squat and the assessments that go along with it. But hey, I'm talking about athletes.
Most people fuck up their backs because they were never taught the correct way to squat and just mimic what everyone else does in the gym. Again, not everyone is structurally built to squat. Here's a great article that hits on several reason why not just crossfitters get hurt but just people in general. This article has a direct carry over to everyone.
https://www.t-nation.com/training/doctors-view-of-crossfit