Well folks, it’s the beginning of a new era: after nearly three decades of KDE desktop environments running on X11, the future KDE Plasma 6.8 release will be Wayland-exclusive! Support for X11 applications will be fully entrusted to Xwayland, and the Plasma X11 session will no longer be included.
Basically X11 lets everything it does talk to everything else it does. There’s minimal isolation and there’s not a clean way to do that in x11 without fundamentally changing how it works and breaking compatibility. There were also other issues to were too messy to solve without breaking things. So it was better to just start over. And now we have Wayland.
One example of this is you could have Firefox and a terminal window up in an x11 environment and Firefox could theoretically see everything you are typing in the terminal window.
I think they wanted to know about the original problem solved by X11