I can never remember what "Py_LIMITED_API" stands for. If it's not defined, does that mean we have the *unlimited* API? Is that a superset or a subset of the limited API?

Regarding 1a *and* 1b, I think it would help to list the specific reasons exceptions and other types are not entirely immutable. Is it just __subclasses__ or is there other state (apart from the refcount) that's mutable and visible to the end user? (Or even if it's visible to C API users.)