DEPR: enforce nested-renaming deprecation
Looks like this deprecation was introduced in #14668 (April 2017), so should be OK to remove by now.

Suggestions for better error messages welcome.

Not passing `_level` around is pretty nice
@jorisvandenbossche i could use some help figuring out what the intended post-deprecation-enforcement behavior is.  From the test failures, I'm guessing some of these cases should not raise? @jbrockmendel can you give an example of a case you are unsure about? > can you give an example of a case you are unsure about?

Since the test failures have been fixed, the remaining question is if there are better error messages we should give > can you add the whatsnew for this in removals & update the deprecation removal issue.

added (but not checked off) to #13777 thanks!
