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Most calendars have the option for you to subscribe to an ical by pasting in the web address. Which they will then keep in sync.

I might be missing something here but thet "Calendar API" exists for a long time: CalDav.

You can have .ical for a whole Calendar, not only an (offline) event. The calendar can be synced with new events, through CalDav, when they are added to the calendar you shared. You would still have to accept the prompt once but that is it.

You're looking for SOGo. https://en.m.wikipedia.org/wiki/SOGo

I track sport events with Fantastical’s interesting calendars.