If the set of slots gets extended, extensions would have to opt-in to use the newer slots, i.e. the availability of slot numbers should be conditional at compile-time.

Returning 0 for slots not supported in a version seems fine to me as well (after reconsideration), as this is also what you would get if you just recompiled the old type with the new Python headers (i.e. all fields added at the end are 0-initialized).

As for slots added to 3.4: it would certainly possible to add them to the stable ABI, if we indeed trust that they are stable (i.e. won't go away until 4.0). That would have to be decided on a slot-by-slot case, preferably in individual roundup issues.