I got tired of having to register and having friends having to register accounts just for accepting a party or dinner invitation (also plenty of friends have left social media) so I coded a minimal, no-login event tool: https://yupno.com
Dev-friendly stuff: Full REST API, async webhooks for RSVPs with Slack/Discord-adapted payloads. Auto-generates Ical, JSCalendar and JSON-LD. No auth, uses admin/user tokens in url. Should be possible connect to other stuff in fun ways.
Free to use, no ads, no tracking. I just wanted a simple tool that respects my time and privacy. Might figure out some way to monetize it if it gets traction, perhaps Stripe integration for tickets to events or something.
Docs/API details: https://yupno.io/docs.html