The relevant term of Article 20 of the directive seems to be "personal data", which means information relating to a person (thus being broader than the idea of

The relevant term of Article 20 of the directive seems to be "personal data", which means information relating to a person (thus being broader than the idea of "personally identifiable information"). If you ran the Facebook page for a club or society, then much of the metadata about the events you created for it would likely not be covered by the GDPR; but for a personal Facebook page, if you were publishing information about an event that you were organising and were planning to attend, it is more likely that a useful amount of metadata would be covered. I don't know whether ActivityPub has a standardised way of handling event invitations and RSVPs, but the existence of things like the iCalendar format and its various implementations might go some way towards making the "technically feasible" argument.