RAP 2.3 is backwards compatible to 2.2, 2.1 and 2.0 with the exception of breaking changes in the (provisional) Remote API. This API had been introduced in 2.0, but announced and declared as provisional. The change affects custom widgets only. It has been announced in the newsgroup and in the 2.1 New and Noteworthy.
The lifecycle package org.eclipse.rap.rwt.lifecycle has been marked as deprecated in an effort to remove the internals of the request processing (lifecycle and phases) from our public API. It will be removed in RAP 3.0.
There was no API removal in RAP 2.3.