You can use `xdpyinfo` yourself to see all the extensions you have loaded.
As for the quality of the code, all you need to do is read the discussions by Xorg maintainers about the quality of Xorg's code back when Wayland was beginning development. You will not find even one optimistic take. There was even an early attempt to fork Xorg and fix it (called X12), but it was quickly abandoned as unfixable.
Note, also, that these discussions began in 2010. More than half a decade before Flatpak or Snap. Work had begun to replace the mess before we were even talking about sandboxing.
As for the quality of the code, all you need to do is read the discussions by Xorg maintainers about the quality of Xorg's code back when Wayland was beginning development. You will not find even one optimistic take. There was even an early attempt to fork Xorg and fix it (called X12), but it was quickly abandoned as unfixable.
Note, also, that these discussions began in 2010. More than half a decade before Flatpak or Snap. Work had begun to replace the mess before we were even talking about sandboxing.