If there's a strong feeling that we should keep the old APIs around, we can do that, but I think that (1) if we don't remove them now, we probably never will and (2) they are vile APIs.
Yep.
Delete 'em.
Things regularly change between releases in ways that're visible to extensions, though not necessarily as obviously part of the extension API. The most obvious being at least one change to ProcessUtility_hook and probably other hook function signature changes over time.
It's a pain to have to #if around the differences, but better to export that to extensions than *never* be able to retire cruft from core. Lets not be Java, still stuck with Java 1.0 APIs everyone knows are unspeakably awful.
Delete the APIs, relnote it with the required changes and be done with it.