BUG / CoW: Series.transform not respecting CoW
- [x] xref #49473 (Replace xxxx with the GitHub issue number)
- [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
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] 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.

Could think about back porting, but I wouldn't do it at this point
No objection here - mainly because the fix is so simple. But in general we don't support UDFs that mutate the object:

https://pandas.pydata.org/pandas-docs/dev/user_guide/gotchas.html#mutating-with-user-defined-function-udf-methods

If we want to revisit this then I'm certainly open, but otherwise I don't think we should go out of our way to support this anti-pattern.