I don’t know if any of my code is still in Kwin but I helped with the standardization effort for X11 window managers to support synchronized window resizing: https://specifications.freedesktop.org/wm-spec/wm-spec-1.3.h...
That was a reasonable hack 18 years ago but a fully composited desktop is table stakes in 2022.
I don’t know why you think it is not the fault of X. X doesn’t care about when will a given program render to the screen. Sure, there are hardware solutions to the problem, as well as compositors, but in the latter case X will have no use other than being a bad IPC.
Did you start using Linux last month? Because I have flicker-free, nicely-animated experience on X for years. Granted it's not experience that everyone has, but it's not fault of X, it's bad drivers or low power hardware. Wayland only brought forced vsync.
GTK4 is actually trash that ruined font rendering because GNOME developers felt so.