I’ve been reading about that, so this is not just an Ubuntu MATE issue, it is becoming a trend on other flavors.
As far as I know from what I could find from official sources:
Lubuntu is effectively in “maintenance mode” as they say on their blog:
We had substantially less development manpower than we’ve had for previous releases, thus our primary focus has been keeping Lubuntu working as well as it has worked historically, rather than innovating new features.
Ubuntu MATE… well, we all know. It’s always been a one-man effort. And the man apparently has moved on.
Even Kubuntu is facing some issues, as the main developer has been facing some difficulties both personal and job related (check out her blog for details).
I don’t know about Xubuntu, as I couldn’t find an official blog or some place that reports on details besides the new releases.
But not everything is lost: remember that years ago, Edubuntu was discontinued and a few years later got back to life.