AFAIK GTK-2 stopped being officially supported as soon as GTK was released. Though there does still seem to be activity in the git repo https://gitlab.gnome.org/GNOME/gtk/tree/gtk-2-24

In any case MATE migrated to GTK-3 some time ago, as of MATE 1.18 https://mate-desktop.org/blog/2017-03-13-mate-1-18-released/

Oh, and GTK-4 is coming soon...

Not sure, but it looks like Mate was ported from gtk2 to gtk3.

https://wiki.mate-desktop.org/status:gtk3

As far as I understand, it is using GTK+3 but following the GNOME 2.0 Human Interface Guide.

MATE supports GTK 3 only as of 1.18, i.e. two stable release ago.