Leaving aside any question of standards or acceptable alternatives, as
far as I'm concerned, we should all use YYYY-MM-DD because it's the
only format that's sortable.
Only if the only sorting option available is an alpha sort and it can only
be applied to the filename, but we don't live in a world like that, do we?
I know what you meant, but what you said is clearly untrue. Any date format
that you can think of, (and document!), is then sortable. Perhaps not by a
simple alpha sort, which I assume is what you meant, but's that's by far not
the only sorting method available.
In a previous life, one of my tasks was to take data files from different
sources, each containing millions of customer records, and normalize the
data. That meant recognizing all the different ways that phone numbers can
be written, the different ways that a state can be referred to, (fully
spelled out or abbreviated), and of course all the ways that dates can be
written. Months that are written fully, as in January, February, and so on,
can be easily sorted, just as they can if they're written as a 3-character
abbreviation or a 2-character (JA, FE, MA, AP, MY, etc.), or as a series of
numbers. Years written as 2 digits were converted to 4 digits, etc. None of
it was rocket science.