This was a feature plugin that never made it into WordPress core. It would have been a great foundation to further build out CMS capabilities and probably many other things that we haven’t even imagined yet.
But, the core developers don’t need to imagine everything that people will do with our CMS, we just need to provide a solid base for people to build on.
I see this API as mainly an un-opinionated way for plugins like ACF, Toolset, etc to do their job, but much more easily, without having to re-invent almost every piece of the functionality they provide.
For developers, this would also allow us to easily register all custom post types, fields, etc in the theme or plugin code for a site. We would change the field definitions in our dev environment, test and deploy, and everything in production is already updated. We could build a simple UI to manage only the settings that really need to be changed, but this UI wouldn’t let users break the custom content types like what happens on sites today.
For users, it’s hard to tell how this would look. I think this would mostly be a back-end improvement, perhaps with some light UI controls, but most user-facing implementation would be up to plugins until clean standards emerge.