Preferably not just ones who got worse material or fell into playing a parody of themselves but ones who seemed to lose their skill in a more inexplicable way. Or, they are incapable of giving a performance without trying to turn it into an event. They can't simply read the script in context and modulate their performances to that.

They also could've gotten worse because they were either:

Felled by sickness

Felled by hype (https://tvtropes.org/pmwiki/pmwiki.php/HollywoodHypeMachine/ExamplesThatDidntHoldUp) crashing back to reality

Felled by poor career choices

Felled by being difficult to work with