Mobile team added MDL-56307 to meet their single very specific requirement for finding if anything has changed in a module for a specific user since a specific timestamp. This is a 90% overlap with the functionality of the existing recent activity API which we already wanted to upgrade to a modern API.
Now we have 2 APIs that must be implemented by every module and can return inconsistent information. The information in the recent activity block and the mobile app is already different as mobile team is only adding support for activities with their new api function AS they implement offline access in the mobile app.
This is very short sighted.
I would suggest we ADD information to the new check_updates API so that it can be used by the recent activity block in Moodle and deprecate the recent_activity API completely.
I am setting the priority on this issue high because it is a very recent mess we have created.