I've never heard of such a contract clause. Care to share a source?
Fan knowledge of this general topic probably began with a review of ST:TMP by Harlan Ellison in Starlog
magazine (April 1980), in which he excoriated Shatner for counting lines in the "City" script. Ellison might also talk about it in a youtube video somewhere; I can't recall for sure. But he obviously didn't understand the legitimate problem faced by TV's lead actors in the 1960s, and didn't bother to find out.
I think I read "official" confirmation that Shatner had one of these role-protecting contract clauses in Inside Star Trek
, of which I read a borrowed copy so I can't find the reference. But it's a pretty well-known thing now.
That hardly sounds like it's "common" though. That's just a Star Trek specific example. I don't recall reading any such thing in Inside Star Trek, but my copy is out on loan so I can't check right now.