The next GTK 3 release, 3.24.36, will drop support for building with Autotools.
GTK has been using Meson in its main development branch since 2016, and in the gtk-3-24 branch since 2018. While we did not officially support the Meson build in the 3.x branch, we always tested it as part of our CI pipeline. Maintaining two build systems in parallel is not without cost, so we are dropping the older Autotools build in favour of the newer Meson one, now that we are confident in its functionality.
Downstream distributors have already started using the Meson build, as it’s faster and easier to use than Autotools on the list of our supported platforms. The artefacts of both builds are identical, except for the release archive created by Meson not containing generated files and a pre-built snapshot of the documentation.
If you are still using the Autotools build in your GTK 3 packages or CI pipelines, we strongly recommend you switch to Meson; if you experience any regression, feel free to open an issue on the GTK issue tracker.