Describe the issue

Summary

In Matplotlib 2.1, the behavior of reusing existing axes when created with the same arguments was deprecated (see #9037). The request is to finally remove this behavior.

My immediate motivation is that I have a collection of projection classes that take as a keyword argument an instance of astropy.coordinates.SkyCoord, which as of Astropy 4.1 are no longer hashable (see also astropy/astropy#9857). This unexpectedly broke my code because I had not realized that projection keyword arguments had to be hashable.

Proposed fix

Remove the AxesStack class, remove the Figure._make_key method, and update the documentation to reflect the removal of the old deprecated behavior.