The developers of the library GTK announced the designation of the X11 protocol backend as outdated and their decision to discontinue support for X11 in the upcoming GTK 5 branch. However, work on X11 systems will continue in the existing GTK 4 branch.
The decision to retire the X11 backend is attributed to the obsoletion of the X11 protocol and the challenges associated with it – the backend is being maintained on a residual basis as the current GTK developers are focusing more on Wayland. The outdated X11 backend hinders the implementation of new features in GTK. Efforts were made in 2022 to find individuals interested in supporting the X11 protocol in GTK, but no suitable candidates were found (source).
Along with X11, the Broadway Backend is also outdated, which enables GTK library output in a web browser window, along with the Shortcuts Window class for displaying keyboard shortcuts and screen gestures. A replacement for GTK Shortcuts Window will be included in the upcoming major release Libadwaita.
It is also worth mentioning the recent publication of GTK 4.17.4 in the testing branch 4.17, which is developing functionalities for the future stable branch 4.18. The new release includes an experimental backend that allows GTK applications to run on Android-based smartphones. An APK package containing a demonstration application is available to explore the capabilities of GTK on Android devices.
In GTK 4.17.4, the DRISTING engine “GL” using OpenGL has been removed. Instead, a new NGL engine has been introduced since GTK 4.14, providing an abstraction layer for OpenGL that operates on top of Vulkan. However, the NGL engine no longer supports systems with outdated drivers and hardware.