@publicvoit Spoiler: The tendency is indeed getting into the direction of not asking for any remote software in the Fediverse to parse RFC-5545 or https://schema.org/Schedule rules for event recurrence. But that shouldn't mean that rules are available, sometimes it's maybe nice to have an immediate hint like "this event happens every friday at 3pm, expect...".
The only way an occurrence can have a stable ID is via the ID of the event that defines the occurrence rules and the occurrence start date and time, best in utc and local time to take into account timezone changes, and maybe the sequence, .i.e. the revision number of the recurrence rule-set.
And now there comes RSVPs, i.e. the intent to be marked as a participant in an event. Here a problem was clearly visible, which is also true for non-recurring events: is a RSVP targeted towards just an event (so just its ID), or maybe further to a specific start time, or location, or even price offer, or more?