registry.md says that time zone IDs are "A valid time zone identifier" with links to LDML and TZDB.

However, LDML and TZDB define two different standards for time zone identifiers: a BCP-47 ID and a longer form string ID. The longer form IANA time zone ID is generally more well understood.

If we keep this function option (#961), it should be restricted to one set of identifiers, probably the TZDB identifiers.