https://docs.python.org/3/library/stdtypes.html#old-string-formatting still lists the 'u' string formatting option, described as "Obsolete type – it is identical to 'd'." and linking to PEP 237.

However, testing indicates that Python 3 does not support a 'u' option and my archaeology suggests that such support was removed in Python 2.4. It seems this has flown under the radar for quite some time.