When Time Comes For GTK5, It Might Be Wayland-Only On Linux
It's important to stress that GTK5 will likely be a long ways out with GTK4 still quite young and fresh, but this past week GNOME developers began thinking about doing away with X11 support in that next major milestone.
Red Hat's Matthias Clasen opened an issue on GTK entitled "Consider dropping the X11 backend" with the simple explanation of: "It is not getting any better, and Wayland is widely available."
In the comments it was reaffirmed that this would be for GTK5 and not a GTK4 update. It was also noted by upstream developers that the X11 code sees little activity.
Emmanuele Bassi commented, "If the "handful of environments" cover 90% of the user base, I would not talk about "massive narrowing" as much as a reallocation of the efforts of a volunteer-driven project. In other words: talk is cheap, code does not write itself, and code that is not maintained is bound to degrade over time. The obvious issue is that X11 isn't getting any functionality, and GTK has already moved towards Wayland as the primary design for features and API already. This means that the X11 backend will either not get any new functionality that application developers will very much rely on, or will act as a barrier for implementing that functionality in GTK."
Unless there is an influx of developers actually contributing X11 improvements to the GTK toolkit code, it's quite likely that when the time comes for GTK5 that back-end could be outright removed.
Red Hat has been working on other preparations for a predominantly Wayland-only future such as XWayland rootfull for running complete desktops under XWayland, among their other upstream modernization efforts for enhancing the Linux ecosystem.