Nope, the new UI concept needed changes to the REST API. These changes are not going to be backported to openHAB 2.x core.
There is also the introduction of authentification layer and user roles.
Removal of ESH references had to be done cause the ESH project is archived and no longer maintained. This has nothing to do with Kai leaving as the project lead.
And that’s definitely not the only reason for a major release, as you can read here
The Road Ahead
Having the framework code under our control, I think it is a good moment to start planning how openHAB should evolve in future. There were many discussions in the past about the bad UX especially for newbies, with too many different UIs and options and the tricky balance between textual and GUI-driven configuration. Imho we should not make any fundamental changes to openHAB 2, but we should start talking about an openHAB 3, which can bring simplifications by reducing complexity (like retiring UIs, removing Xtext from the core, etc, tbd.) with the cost of breaking backward compatibility (while keeping a migration path for everyone).