There is a MATE fork called CAFE, which uses the CTK toolkit (forked
from GTK3). Seems there has been quite a bit of work in progress over
the past year, and there are DEB files provided upstream. I have tested
this on a VM with the forked packages, and it's running decently stable.
I think this would be a good addition to Debian's repositories for
anyone who likes MATE but isn't optimistic about GTK's future. Please
take a look and consider its inclusion. Thank you.

Upstream: https://github.com/cafe-desktop