Once the Timestamp library is updated per T224729: Tag a new release of Timestamp and update MediaWiki to use it, we'll need to update the Action API documentation and send an announcement to the mediawiki-api-announce mailing list.
Specifically, the documentation in the api-help-datatypes i18n message needs to be updated to reflect the fact that timezones are no longer ignored, and that ISO 8601 dates with timezones are now accepted.