The ui_init extension point had several issues and always was kind of a hack. Most significantly UI changes done through this endpoint would not be undone when the plugin got disabled.

We should replace this with explicit extension points in the new Plugin API to extend specific parts of the UI cleanly.

If plugins would want to directly modify the UI in some other way on start the new enable / disable hooks on the plugins will provide a better experience, as the plugin has the power to undo its changes. However, such direct manipulation of the UI is still discouraged.

The ui_init extension point also was not officially documented.