The option was introduced in the coreutils
date (which is probably what you have) in 1999 (Apr. 8).
The documentation was removed in 2005 without much explanation in the commit.
In 2011, the help for --iso-8601 was reintroduced with the following explanation:
We deprecated and undocumented the --iso-8601 (-I) option mostly
because date could not parse that particular format. Now that
it can, it's time to restore the documentation.
* src/date.c (usage): Document it.
* doc/coreutils.texi (Options for date): Reinstate documentation.
Reported by Hubert Depesz Lubaczewski in http://bugs.gnu.org/7444.
It looks like the help was taken out in version 5.90 and put back in, in version 8.15 (it is not in my 8.13) and the comment above suggests that it is now back to stay and not likely to be disappearing any time soon.
In version 8.31 (as provided by Solus July 2020) the man page descriptions for two two options are:
output date/time in ISO 8601 format. FMT='date' for date only (the default), 'hours', 'minutes', 'sec‐
onds', or 'ns' for date and time to the indicated precision. Example: 2006-08-14T02:34:56-06:00
output date/time in RFC 3339 format. FMT='date', 'seconds', or 'ns' for date and time to the indicated
precision. Example: 2006-08-14 02:34:56-06:00