Output of: datetime.datetime.now().strftime('%D')
is equivalent for: datetime.datetime.now().strftime('%m/%d/%y')

Is there a reason that directive %D is missing in documentation? Are there other hidden directives?
https://docs.python.org/3/library/datetime.html#strftime-and-strptime-behavior

It's dependent on the platform's C library's strftime(). I think (but don't promise) that we document the C89 codes, but each platform might provide additional ones.

Perhaps it could be mentioned more clearly in the datetime documentation that apart from the directives of the C89 standard, you can also use those that the underlying libc strftime() supports.

The following sentence from https://docs.python.org/3/library/time.html#time.strftime describes it well in my opinion:
"Additional directives may be supported on certain platforms, but only the ones listed here have a meaning standardized by ANSI C."

Perhaps a reference to "man strftime" would also be helpful.

A reference to “man strftime” was added in b7117af, and moved to the current place (below the table) in #13410 -- before this PR was filed.
The sentence just above the table also says that only standard format codes are in the table.

I don't think there's anything more to do here.