For example "Message" is using http://xmpp.org/extensions/xep-0091.html which is obsoleted in favour of http://xmpp.org/extensions/xep-0203.html.
MessageEventExtension --> ChatState
...
This needs a list of extensions and features that should be fixed, dropped and implemented.
Once initial lists are available, votes and comments for them could be added.
Other than that, Message has now support for both legacy (0091) and the "new" (0203), transparently for the user/developer.
ChatState should be considered a MessageEvent that contains a message with state that has value other than null.