Debian still ships support for token ring; the idea that X11 is ever "going away" really misunderstands how free software works.
However, development of xorg is completely dead so it is pretty likely that distributions will stop support running xorg sessions in the future, and at some point xorg will likely not run on new graphics cards anymore.
KDE has announced plans to discontinue support for running x11 sessions and only runs on wayland in the upcoming plasma 6 version.
For now all the gui toolkits will presumably continue to support x11, but at some point, it is likely that new applications will only care about supporting wayland, and after that, it is possible that future versions of gui toolkits will stop supporting x11.
Maybe "ripping out working code" is a bad thing, but at a certain point people would have to put in extra effort to ensure that x11 stays "working" and if nobody steps up to put in that effort and x11 support breaks, it won't be ripping out "working" code anymore.
I don't think that's true; didn't it just recently get a new maintainer who put out a bug fix release and everything?
- fractional scaling of the display output
- app rendering at fractional scales.
The first one works fine with Gtk 3 and newer; the application renders its output at nearest higher integer scale, and then the compositor downscales it to requested fractional scale (i.e. app renders at 200% and compositor downscales it to 175%). Apple does exactly this same thing (just with slightly different scales; they won't show you nice 150% or 175% for a reason; they optimize for different thing).
The second one is rendering at fractional scales directly by the application; then the compositor doesn't downscale anything, just displays the buffer as it is (i.e. app renders at 175% and compositor displays it as it is). Since rendering at fractional scales is more demanding (just think about fractional mouse coordinates or how to display 1px straight line and don't get lost in the rounding errors), it took longer to materialize. It is this support that will have to wait for Gtk 5.
But meanwhile, your (Gtk) apps will be displayed at fractional sizes using the first method.
I do not think this is current info, but otherwise might be the reason as it is an experimental feature.
For example, you can enable fractional scaling in GNOME right now.
https://www.youtube.com/watch?v=BjfZ8TSXsps
This phoronix article shows the variable to enable it in GTK.
https://www.phoronix.com/news/GTK-4.11.1
Edit: added a little clarification + cited the specific bit of text I was responding to.