DEPR: DataFrame.groupby(axis=1)
- [x] closes #51203 (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Any more thoughts here? @jbrockmendel - thanks for the ping; just a conflict with the whatsnew. Thanks @jbrockmendel  nb  the 2.1.0 docs at https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.groupby.html do not mention this depreciation Thanks @cfaaron - I've opened #54873.