I wouldn’t speculate on how common it is but limiting passwords seems to happen more than it should. So maybe many are taking the stealth approach.

One site I know where this happens (at least I experienced it some years ago) was Blizzard. Found out by sheer luck after I clearly fumbled the end of my password and was logged in regardless.

No, that’s the point, you’d never know whether they only validate a subset of the password. Only by testing different variations you would know that less than the whole string still works.

How would you know?