DEPR: sort=True in DataFrame.unstack and Series.unstack

[rhshadrach]

The sort argument was added in #53298 because of #15105. Now that users can get the unsorted result, they can use sort=False and call .sort_index after unstacking if they wish to have sorted columns.

[jorisvandenbossche]

The keyword was only added in main (i.e. not yet released), so there is no need to deprecate it first if we still want to change it?

Or it's the default of True that you want to change to False eventually?

[jorisvandenbossche]

And this is only for unstack, right? (now that the sort keyword has been removed again from stack in #53825)

[rhshadrach]

Or it's the default of True that you want to change to False eventually?

Yes - the goal is to make sort=False the only behavior. So we would deprecate both not specifying sort and specifying sort=True. Then in 3.0, sort would only be able to be False, and we would deprecate the argument entirely.

[rhshadrach]

Clearing the milestone, there are currently issues with sort=False. See #55516.