DEPR: Change FutureWarning for observed=False to DeprecationWarning
- [ ] closes #54970 (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.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

https://github.com/pandas-dev/pandas/issues/54970#issuecomment-1730686981 makes me think more that we should not revert. Yeah I think this is a case where we should have been more careful in hindsight misjudging the "impact", but I don't think we should change to a `DeprecationWarning` due to the churn.