*Server* grabs are much much rarer (and much more dangerous) though. But, yes, keyboard and (especially) mouse grabs are ubiquitous -- and occasionally a buggy app refuses to let go. Hence the usefulness of a break-it option. And the config options of discourse only let you break keyboard and mouse grabs anyway, not server grabs. There is no such thing as a 'screen' grab in the X protocol, as far as I know.
(Aside: I kind of wish that grabs were time-limited unless clients explicitly lifted the limit. That way, screen savers could lift the limit, and everything else would get sanity restored automatically if they buggily failed to release a grab, rather than ruining everyone's day. What you do if a grab expires is another matter. Send a new event, I suppose: killing the client is too evil and emitting an X error is nearly useless because they're too asynchronous so nobody handles them in any way except to kill the app.)