I'm afraid I was only the main contributor to Ubuntu MATE's Welcome, Boutique and website many years ago (more of the web developer). I wasn't involved 
in MATE development or QA testing. I didn't upgrade past 16.04 LTS (GTK 2).
I completely agree that Ubuntu MATE truly feels like the Ubuntu flavour of MATE. 
+ 
. The lagging update (MATE 1.28) is because Debian hasn't got it due to a technical version problem.
If we think of teams, MATE is still live. Repositories are still seeing activity. I guess it's the nature of "mature & stable" software. Things like Wayland are big tasks so I can understand if it's not something developers feel like passionately working on in their free time after presumably a day of work.
The 
Ubuntu MATE team? ... Who?... I personally feel like the flavour is coasting down the road. We don't hear much from the leader. A lot of contributions over the years is what makes up the team page, but maybe it's just quiet / less interesting these days. It's not dead, because we have a solid moderation team, and this community is our asset.
Is Lubuntu still alive? A similarly small flavour too. Because their forum closed and merged with Ubuntu Discourse. It wouldn't surprise me if Ubuntu MATE got a similar fate.