Follow up on #22329 to add a deprecation cycle for this API change and avoid breaking dependent libraries! :)

PR Summary

The following will now raise a MatplotlibDeprecationWarning instead of RuntimeError:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
l = ax.axhline(y=0.5)
l.set_ydata(0.25)

PR Checklist

Documentation and Tests

• Has pytest style unit tests (and pytest passes)
• [n/a] Documentation is sphinx and numpydoc compliant (the docs should build without error).
• [n/a] New plotting related features are documented with examples.

Release Notes

• [n/a] New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• [n/a] API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst