I also wonder how Mate is going to adapt: the titlebar-less maximized windows are among its official supported features.
Not one in Mate is develop Gtk, so in theory they can no do anything, but in practice the support for that feature can be reintroduced as a distro specific patch to Gtk (probably for all distributions that want to fully support the Mate desktop). This is of course complex and is not fully on the hands of the Mate developers. Also i’m not saying here that the Mate team will used that idea. What i said it’s just that, an idea. Also I said that only because it’s how this was resolved before in similar situations.
What catches my attention is to see how there are external people who completely omit that Linux is more than a particular environment (for example: GNOME) and they try to give recommendations that can probably improve the user experience in a particular environment, but undoubtedly this will break the user experience as a whole outside that particular environment.
If all people would realized that there is a world out there somewhat farther from where they reach their sight, we probably had a better ecosystem in Linux.