Comment by OkGoDoIt

At least the calendar aspect of this has existed for over a decade. Similarly there are standardized protocols for handing sever-based contact lists and email and chats. Sure most of those protocols are crappy but you’ll just use a library anyway so that’s not an excuse. The problem is not on the technical side. Basically all the big providers have decided a walled garden is the best long term strategy and have been dropping support for any standards-compliant data sharing.

14 years ago I was syncing my contact lists from my personal server to a flip phone over vCard format (I think? Don’t remember exactly, but the contacts equivalent of CalDAV). Nowadays you either need to oAuth into Google or whatever other centralized provider a user might have, with whichever proprietary API they may or may not give you access to.

As with so many other things, a shiny new technical solution won’t change the reality that big companies are not incentivized to support interoperability.