ENH: dont guess %Y/%m

[MarcoGorelli]

Follow-up from #49389

Guess "year month", but only if it's "%Y-%m" as that's ISO8601 compliant

[mroeschke]

Just curious, is the intent to eventually only guess formats that are ISO compliant? I think removing this functionality is okay if we can justify (document) to the user why

[MarcoGorelli]

The goal would still be to guess most date formats, especially non-ISO8601 ones

It's just for dates which don't have all three of "year, month, and day" that perhaps it's better to not guess (unless they're ISO8601), cc @WillAyd

[WillAyd]

Yea I think adhering to ISO for inference is going to be the easiest to develop / communicate.

[mroeschke]

Okay since this is kind of an edge case I am okay with this then as long as it's stated somewhere (in this or a following PR) that some prior formats are no longer guessed since they were not ISO

[WillAyd]

Thanks @MarcoGorelli