As per the discussion in
https://github.com/tc39/proposal-temporal/issues/2379#issuecomment-1248557100
and the PR https://github.com/tc39/proposal-temporal/pull/2398, which is
to be presented for consensus to TC39 in the upcoming plenary meeting, UTC
offsets and the Z designator should be disallowed after any date-only
strings (YYYY-MM-DD, YYYY-MM, and MM-DD). They should only be allowed to
follow a time component. Z remains disallowed in any string being parsed
into a Plain type.
Annotations become allowed after any ISO string, even YYYY-MM and MM-DD
where they were previously disallowed.
Philip Chimento