I can emphatically tell you that I have a mutual non-disparagement clause with [REDACTED]; _and_ that these clauses seem to be routine enough to be included with most "resolution contracts", at least in Europe (<your favorite search engine here> results seem to indicate that these clauses are very popular in Europe).
Most people don't have the equivalent resources (that business have) to fight these clauses; especially, where the resolution - itself - may haven taken so long that the person is essentially "glad" to have some modicum of restitution and sense of resolution.
This means/infers that these clauses (generally) are de facto enforced by "better safe than sorry" scenarios.
(Generally, whether they're legal and enforceable comes down to the clauses first having been breached and, then, being challenged in court. This means/infers effort, time, and money that the people these clauses tend to - generally - target don't have.)