But that we have. Well, you would have to make two calls, since purgeAll only takes a single class, but provided the index only contains A and B and you purge them both, I'll expect the index to be empty afterwards.

No we don't have that today.
Imagine an index that has A, B and an old C or even some D data put in there by Lucene access directly.
If you purgeAll(A.class);purgeAll(B.class); then C and D are still in the index.
With my proposal, C and D would be gone too.