DEPR: dropping nuisance columns in DataFrame reductions
- [ ] closes #xxxx
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Discussed on this week's call
added a whatsnew subsection.  this is actually just the first half of the note im about to push for #41475.  

the smart money says ive made a mess of the rst conventions regarding `code-block:: ipython` vs `ipython:: python` vs [...] ping