I've put up
gh-12945 that preserves forward compatibility of NumPy after the addition of timsort by reusing the mergesort slot in PyArray_ArrFuncs for stable sorts in general, not just mergesort. The same method can be used when we add radixsort for integer types. See the discussion at the PR link for more details on the nature of the problem and considerations for the dtype proposals. I'm waiting on agreement that this is the way to go before proceeding.