ssh -X (or -Y) forwards your X11 connection automagically. It doesn't work for Wayland because Wayland was designed to be impossible to forward.
However, most X11 apps will have utterly abysmal performance on WAN SSH connections. Ancient apps will be slow because they block waiting for every response (this is unavoidable in Xlib) and less than ancient apps will be slow because they send full pixel data. It is possible to write a reasonably performant remote X11 app (using XCB instead of Xlib and always being aware of latency), but there are very few such apps. This is why Wayland dropped the requirement.
ssh -X (or -Y) forwards your X11 connection automagically. It doesn't work for Wayland because Wayland was designed to be impossible to forward.
However, most X11 apps will have utterly abysmal performance on WAN SSH connections. Ancient apps will be slow because they block waiting for every response (this is unavoidable in Xlib) and less than ancient apps will be slow because they send full pixel data. It is possible to write a reasonably performant remote X11 app (using XCB instead of Xlib and always being aware of latency), but there are very few such apps. This is why Wayland dropped the requirement.