DEPR: Index inferring numeric dtype from ndarray[object]
- [x] closes #40489
- [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
- [ ] whatsnew entry

> any perf implications?

haven't run any asvs, but i expect we'll pick up a bit in the constructor in the affected corner cases Didn't check how it's related, but it seems it caused a 4x slowdown specifically in `is_all_dates` for MultiIndex and IntervalIndex:

https://pandas.pydata.org/speed/pandas/#index_cached_properties.IndexCache.time_is_all_dates?python=3.8&Cython=0.29.21&p-index_type='IntervalIndex'&p-index_type='MultiIndex'&commits=126a19d0

