My post refers to GTK4 as your quote indicates. The page describing the GTK4 changes has been taken down, but it basically said that traditional applications won't be possible with GTK4.
There is no need for immediate concern. GTK 3.24 will probably be supported for a long time. I have no idea how long, but maybe 10 years?
So your MATE desktop can look the way it currently does unless MATE devs want to push the Gnomification like Xfce devs are currently doing. I don't say this as criticism, because from the perspective of Xfce devs they are probably preparing for the future. They want to ensure that Xfce is compatible with the direction GTK is going even if it is possible to stay with GTK3 for a long time.
Sorry if it sounded like an emergency. There is probably a 10 year transition window to GTK4.