I'm sad about that too. I have been running MATE with XMonad for a very long time and has been very happy with it, primarily because the experience has remained entirely unchanged except for a few bugs which have crept in and have never been fixed, probably due to the maintainers not really being active. A particularly hilarious one is that every time the update notifier wants to restart the computer it tries to show an icon in the status bar which begins with `un-<...>.png`. The real icon file is actually missing, but some helpful fallback logic defaults to the closest match which happens to be a 512 pixel wide logo for the United Nations which gets vertically cropped and placed in the status bar.
I just got a new laptop this month and saw that Ubuntu MATE was probably going to be unsupported, so I switched to Manjaro Sway.