DEPR: deprecate Index.is_categorical
- [ ] progress towards https://github.com/pandas-dev/pandas/issues/50042
- [x] [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/v2.0.0.rst` file if fixing a bug or adding a new feature.

@topper-123 Does this one seem ok as well? You need to make the test suite pass, prpbably there’s some usage if this method in the code  base. Ping when you make it pass.