Deprecate joining over a different number of levels
- [x] closes #34862
- [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

I think deprecating this is a good idea, I stumbled across this accidentally this week and found this really odd
lgtm ping when updated thanks @phofl 