Hi, I'm just a regular user so this is just an opinion, or rather a guess: All the core components of the MATE desktop, even those that are now absent from Ubuntu MATE continue to be actively developed, since other distributions rely on them and as far as I know they don't use Appindicators so would need the Notification Area.
Besides, in my updated Ubuntu MATE 18.04, Appindicators are used alongside Notification Area in the default layout (they are both present at the same time), so removing Appindicators from the panel lets Notification Area make use of Volume Control, Network Manager, Battery, and Pidgin.
18.04 looks to be coming along very nicely.
Side note: A panel layout like the one default in Parrot Security OS maintains the traditional menu on top but also uses Brisk Menu on the bottom panel. To me this is the best of both worlds and removes the need for a separate application launcher (besides Alt-F2).