Judging by codecov, assert_str_equal in tests/__init__.py is never used (https://codecov.io/gh/matplotlib/matplotlib/src/master/lib/matplotlib/tests/__init__.py#L22), and grep agrees:

$ grep -r 'assert_str_equal' lib/
lib//matplotlib/tests/__init__.py:def assert_str_equal(reference_str, test_str,

So I don't see why it shouldn't be deprecated and then removed.