That was with the Cynical Calendars Plugin.
Works perfect for years!
But unfortunately Apple has broken the API call.
Apple has a habit of deprecating APIs, replacing them (kind of, sort of) with new, incompatible ones. That they did with the API I use, some six years ago. Deprecation takes a long time (Apple doesn’t like to just break stuff, they give lots of warnings… but they never relent). That’s what actually limits the lifetime of the plugin
So, it's a pity that at the moment there is no Apple Calendar integration.