alt Hacker NewsIt's not at all a legal grey area.
If it was, people fired or laid off and not offered severance would have standing to sue.
They don't.
Calling it a legal grey area is like calling vaccines a medical/scientific grey area. Or calling perpetual motion machines a physics grey area.
You could say the same thing about the right to abortion access in the US until Roe vs. Wade was overturned (or equivalently, before Roe vs. Wade).
Comparisons to physics are unwarranted.
Laws change, and they are interpreted when applied. Very few things are black-and-white to begin with, and legality of non-disparagement clauses is one of them.