[WorldMaker]

Apple Invites does provide ICS files for the events. (In the web version when not logged in to an Apple Account, after RSVPing.)

Technically vCal/iCal/ICS (whichever name you prefer) doesn't actually support RSVPs. It isn't in the standards documents. In ancient Microsoft nomenclature that pseudo-standard (de facto standard) for RSVPs is the "Schedule+ protocol" named after an ancient dead predecessor to Outlook's Calendar which originated it. I don't know what Google or Apple call it, and it is such a weird dance of (usually) auto-deleted email messages, so certainly has room for improvement as a protocol.

It would be neat to encourage a new "modern" standard there. Seems like something more web-based (JSON REST API?) than email-based might be a more "natural" API today. (Maybe Apple Invite can help lead the way, I don't know if that's on their TODO list.)

[gsnedders]

iTIP (RFC 5546) / iMIP (RFC 6047) are a standard for sending and responding to calendar invites, implemented on top of email.

Certainly some implementations are pretty poor, but in theory this is all standardised.