Ars Technica has quotes from a Canonical employee (emphasis mine):
This is a return to the early years of Ubuntu, when the desktop
shipped with GNOME instead of a Canonical-developed user interface.
Shuttleworth's blog post didn't specifically say that phone and tablet
development is ending. But Canonical Community Manager Michael Hall
confirmed to Ars that the Ubuntu phone and tablet project is over.
"Work on the phone and tablet is also ending, the whole convergence
story, really," Hall said. "The desktop will continue, but like it was
in the pre-Unity days where we took what upstream [developers]
designed and developed."
[...]
By switching to GNOME, Canonical is also giving up on Mir and moving
to the Wayland display server, another contender for replacing the X
window system. Given the separate development paths of Mir and
Wayland, "we have no real choice but to use Wayland when Ubuntu
switches to GNOME by default," Hall told Ars. "Using Mir simply isn't
an option we have."
Since they're now falling back to upstream stuff, there's no reason to expect Unity to remain in development as a beta or any other form.
Also note that development != maintenance.