Many hours have been wasted on calendaring servers, and they still don't solve the problems people who use calendars want solved. This is because the problem is approached from the wrong direction. People think from the client to the server, as it's clients originating ics files which are then schlepped around via email. Servers allowed people to do things like free-busy for attendees and conference rooms, but required email clients to support things like itip. I'll give you one guess how that went.

This model instantaneously breaks down when you go cross-organizational. The widespread incompatibility between mailservers and no standardized way to federate directory discoverability makes this impossible. As such, the meta collapses back to schlepping around ics files. It should shock nobody that embracing this fact and giving up on free/busy and presence has been the solution that dominates. Microsoft has implemented on this approach better than anyone decades ago.

Actually solving the presence problem requires that you get federation right. Guess who's doing that? The chat apps. Both Slack and Teams have this figured out. Doing this as a plugin to matrix or snikket would actually be quite straightforward. As such my recommendation is that shared hosting software stop distributing calendaring software. They should instead distribute chat servers and good chatbots that can do things like meeting reminders.

You could even federate across incompatible chat apps and protocols via bots that know how to talk to each other. I know it would work, because it worked before computers. That's how secretaries coordinated all of it -- picking up a phone. Implementing a UI for people to use would be as simple as wrapping your secretary bot that knows how to book people and rooms.