it’s mostly solved already

I wished I believe this. Or I guess I agree that it is solved in most software but there is lots of commonly used software where it isn't. One broken bit of software can fairly easily take down a whole site or OS.

Try to create an event in 2040 in your favourite calendar. There is a decent chance it isn't supported. I would say most calendar servers support it, but the frontends often don't or vice-versa.