Rather, we might state that "At some point in the future, the NumPy development team may no longer interested in maintaining workarounds for specific subclasses, because other interfaces for extending NumPy are believed to be more maintainable/preferred."

That sentence I think covers it very well. Subclasses can and should be expected to evolve along with numpy, and if that means some numpy-version dependent parts, so be it (we have those now...). It is just that one should not remove functionality without providing the better alternative!

-- Marten