 DEPR: deprecate fastpath keyword in Index constructors 
From all that looking at Index constructors ... 

Apparently it was even hardly used internally for the Index classes (except a little bit in RangeIndex).  

Part of https://github.com/pandas-dev/pandas/issues/20110


Hello @jorisvandenbossche! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/indexes/base.py`](https://github.com/jorisvandenbossche/pandas/blob/a27da847d67e04f7c566c3d9051190d169665894/pandas/core/indexes/base.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/category.py`](https://github.com/jorisvandenbossche/pandas/blob/a27da847d67e04f7c566c3d9051190d169665894/pandas/core/indexes/category.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/numeric.py`](https://github.com/jorisvandenbossche/pandas/blob/a27da847d67e04f7c566c3d9051190d169665894/pandas/core/indexes/numeric.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/period.py`](https://github.com/jorisvandenbossche/pandas/blob/a27da847d67e04f7c566c3d9051190d169665894/pandas/core/indexes/period.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/range.py`](https://github.com/jorisvandenbossche/pandas/blob/a27da847d67e04f7c566c3d9051190d169665894/pandas/core/indexes/range.py) !

 - There are no PEP8 issues in the file [`pandas/tests/indexes/test_base.py`](https://github.com/jorisvandenbossche/pandas/blob/a27da847d67e04f7c566c3d9051190d169665894/pandas/tests/indexes/test_base.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23110?src=pr&el=h1) Report
> Merging [#23110](https://codecov.io/gh/pandas-dev/pandas/pull/23110?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/12a0dc49ac63b63bcc5f93bccf71bce51c60bdad?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `96.29%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23110/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23110?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23110      +/-   ##
==========================================
+ Coverage    92.2%   92.21%   +0.01%     
==========================================
  Files         169      169              
  Lines       50888    50895       +7     
==========================================
+ Hits        46919    46935      +16     
+ Misses       3969     3960       -9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <96.29%> (+0.01%)` | :arrow_up: |
| #single | `42.23% <40.74%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23110?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/23110/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `93.24% <0%> (-0.03%)` | :arrow_down: |
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/23110/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `97.56% <100%> (+0.29%)` | :arrow_up: |
| [pandas/core/indexes/numeric.py](https://codecov.io/gh/pandas-dev/pandas/pull/23110/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9udW1lcmljLnB5) | `97.44% <100%> (+0.03%)` | :arrow_up: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/23110/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.6% <100%> (+0.05%)` | :arrow_up: |
| [pandas/core/indexes/range.py](https://codecov.io/gh/pandas-dev/pandas/pull/23110/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9yYW5nZS5weQ==) | `95.77% <100%> (+0.03%)` | :arrow_up: |
| [pandas/core/arrays/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/23110/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lbGlrZS5weQ==) | `95.34% <0%> (-0.15%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23110/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.63% <0%> (-0.02%)` | :arrow_down: |
| [pandas/io/formats/csvs.py](https://codecov.io/gh/pandas-dev/pandas/pull/23110/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvY3N2cy5weQ==) | `98.21% <0%> (ø)` | :arrow_up: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23110/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `96.94% <0%> (+0.02%)` | :arrow_up: |
| ... and [2 more](https://codecov.io/gh/pandas-dev/pandas/pull/23110/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23110?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23110?src=pr&el=footer). Last update [12a0dc4...4575847](https://codecov.io/gh/pandas-dev/pandas/pull/23110?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 General +1, couple of comments.  Is there a tracker issue for deprecations-to-eventually-remove? yes
search for the label of Deprecation then sort by most comments Added it to the list in https://github.com/pandas-dev/pandas/issues/6581 @jbrockmendel Thanks for the review! @jorisvandenbossche can you update What do I need to update?  > why are u changing these constructors at all?

comment above Answer above: https://github.com/pandas-dev/pandas/pull/23110#discussion_r225005757 thanks @jorisvandenbossche  thanks!