ENH: Index[bool]
- [x] closes #15890
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

> Thoughts on deprecation path?

i think it might be ok to actually just change this, are you finding that you need to change test behavior? >  are you finding that you need to change test behavior?

Yes.  Largely around factorize and value_counts where this branch is currently very ad-hoc. looks ok for 1.4? > looks ok for 1.4?

I'd rather not rush it. > minor comment. ready to go?

just pushed with a fix.  i think ready, yes. can you rebase again (as the scipy fix merged) thanks @jbrockmendel very nice!