Deprecate DataFrame indexer for iloc setitem and getitem
- [x] closes #39004
- [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

> See my note on the PR: [#39004 (comment)](https://github.com/pandas-dev/pandas/issues/39004#issuecomment-756344692)
> 
> In addition, if we disallow this, I think it should be deprecated first for assignment (since that case was working right now)

this is reasonable to deprecate for setitem and simply raise for getitem. @phofl if you can amend.

though I am tempted to simply call this a bug fix if setitem actually gives wrong behavior? (or is it actually somewhat correct?) Don't know if I would call this wrong. It is counterintuitive for sure. 

If you *know* that iloc does not align, you may expect the result, Additionally it only works if indexer has the same dimension as the object to index.

Will add a DeprecationWarning > Will add a DeprecationWarning 

FutureWarning but thanks Added the Warning Hello @phofl! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-03-02 19:53:34 UTC Added a deprecation whatsnew @jreback green Let's give this a bit more time, since the discussion in https://github.com/pandas-dev/pandas/issues/39004 is still ongoing @phofl conflict and if you can rebase. i think this is fine to merge. Done, failure unrelated this looks fine to me. @jorisvandenbossche are you still objecting? @phofl can you merge master and update @jreback merged master, failure unrelated everyone ok here ?

@jorisvandenbossche @phofl @jbrockmendel @gooney47 LGTM Are we good here? @jreback greenish thanks @phofl 