On branch 9222, you can now insert, update, and delete EE4 model data (eg events, venues, registrations, payments… almost all EE4 data) via the REST API interface.
This will facilitate tasks like event registration via a single-page javascript application, updating registration and event data from a mobile app, and synchronizing data from a 3rd party web service to EE4.
It is currently undergoing internal testing, but we invite testing and feedback from others. Please give it a try, and open a github issue if you have suggestions (or heck, even if you have no suggestions, that’s good to hear too; it will help us get it released sooner).