Citing from Debian bug report #967757:
This package has Build-Depends on GTK 2 (libgtk2.0-dev), or produces binary packages with a Depends on GTK 2.
GTK 2 was superseded by GTK 3 in 2011 (see https://bugs.debian.org/947713). It no longer receives any significant upstream maintenance, and in particular does not get feature development for new features like UI scaling on high-pixel-density displays (HiDPI) and native Wayland support. GTK 3 is in maintenance mode and GTK 4 is approaching release, so it seems like a good time to be thinking about
minimizing the amount of GTK 2 in the archive.
GTK 2 is used by some important productivity applications like GIMP, and has also historically been a popular UI toolkit for proprietary software that we can't change, so perhaps removing GTK 2 from Debian will never be feasible. However, it has reached the point where a dependency on it is
a bug - not a release-critical bug, and not a bug that can necessarily be fixed quickly, but a piece of technical debt that maintainers should be aware of.
A porting guide is provided in the GTK 3 documentation: https://developer.gnome.org/gtk3/stable/migrating.html
Some libraries (for example libgtkspell0) expose GTK as part of their API/ABI, in which case removing the deprecated dependency requires breaking API/ABI. For these libraries, in many cases there will already be a corresponding GTK 3 version (for example libgtkspell3-3-0), in which case the GTK 2-based library should probably be deprecated or removed itself. If there is no GTK 3 equivalent, of a GTK 2-based library, maintainers should talk to the dependent library's upstream developers about whether the dependent library should break API/ABI and switch to GTK 3, or whether the dependent library should itself be deprecated or removed.
A few packages extend GTK 2 by providing plugins (theme engines, input methods, etc.) or themes, for example ibus and mate-themes. If these packages deliberately support GTK 2 even though it is deprecated, and they also support GTK 3, then it is appropriate to mark this mass-filed bug as wontfix for now. I have tried to exclude these packages from the mass-bug-filing, but I probably missed some of them.
Citing from Debian bug report #967757: