Most airlines have language in their CoC that reserved seats aren't guaranteed, which usually means that GAs can move people to seat special needs passengers in designated seats or to put parents together with (very) small children. FAMs tend to demand particular seats (aisle) too. I guess these could be cases of politicians and celebrities traveling commercial with body guards, but if they want privacy, they should buy the entire cabin.

If someone is moved into a less desirable seat for any other reason, I would expect the airline to apologize with some customer service gesture.

The stories here tend to suggest that someone is more at risk for this happening to them when they board late. Obviously airlines will prefer to do it to those without status than their own elites or HVCs and perhaps partner elites too.

BTW, I'd be reluctant to mention a medical condition as the airline could respond by demanding medical clearance before you can fly them again.