It’s true that there are some apps not directly associated with any DE that have moved to GTK4. If GTK5 actually was likely to come out any time soon you might have to worry about finding alternatives when they switch to GTK5. That being said though, GTK3 had been out for over 9 years before GTK4 came along, there were 4 years between the “last” version of GTK3 and GTK 4.0.0, they’re still in the “oh, this is what we’ll probably do when we release GTK5” phase of development, XFCE has already made a bunch of progress porting everything over to Wayland, and DE agnostic apps are less likely to switch over right away if mid-size DEs like XFCE and Cinnamon still don’t have good Wayland support. I wouldn’t stress out about it too much.
It’s true that there are some apps not directly associated with any DE that have moved to GTK4. If GTK5 actually was likely to come out any time soon you might have to worry about finding alternatives when they switch to GTK5. That being said though, GTK3 had been out for over 9 years before GTK4 came along, there were 4 years between the “last” version of GTK3 and GTK 4.0.0, they’re still in the “oh, this is what we’ll probably do when we release GTK5” phase of development, XFCE has already made a bunch of progress porting everything over to Wayland, and DE agnostic apps are less likely to switch over right away if mid-size DEs like XFCE and Cinnamon still don’t have good Wayland support. I wouldn’t stress out about it too much.