GNOME’s Nautilus file manager is finally matching the behavior of other file managers like KDE’s Dolphin and Xfce’s Thunar with a keyboard shortcut for copying and pasting files.
This Week in GNOME highlighted a notable albeit one could argue long overdue change for GNOME Files / Nautilus: Ctrl+Insert and Shift+Insert support for copying and pasting files.
GTK 5 is not yet in the works to my knowledge.
GTK 3 apps support Wayland natively. GIMP, for example, is GTK 3 and Wayland native.
GTK 2 is X11 only if you use a distro that even ships GTK 2 anymore. Many do not, including mine.