Fair enough. But for that purpose, we need to consider each call into
Lisp, either directly or via a hook, as potentially triggering GC.

Moreover, if some code can signal an error or throw to a higher level,
that could cause GC via the handlers installed by the various
unwind-protect forms. So signaling/throwing are also GC triggers, at
least in some situations, and I'm not sure how relevant that is to
what you had in mind.

(People also tend to forget that GC doesn't only deletes "garbage"
objects, it also has other potentially "surprising" effects: it can
compact strings, relocate string data and buffer text, shrink regexp
pattern cache and font caches, etc.)