gtksourceview3 was superseded in 2018 by gtksourceview4 (a new major
upstream release, but still using GTK 3). There's a porting guide here,
which is mostly "stop using deprecated APIs" plus a few function renames:
https://gnome.pages.gitlab.gnome.org/gtksourceview/gtksourceview5/porting-guide-3-to-4.html

gtksourceview3 is unmaintained and hasn't had an upstream release for more
than 5 years, so it would be great if the remaining gtksourceview3
applications can move to gtksourceview4 before trixie.

I was surprised to see that mate-applets has a build-dependency on
libgtksourceview-3.0-dev, but no runtime dependency: is its use of
GtkSourceView working correctly? Or if it isn't actually needed, then
perhaps the dependency could just be removed without losing functionality?

gtksourceview4 has itself been superseded by gtksourceview5, which is
for GTK 4 applications. Ideally GTK 3 applications should move to GTK 4,
but that's a larger and more intrusive change. Porting guide:
https://docs.gtk.org/gtk4/migrating-3to4.html,
https://gnome.pages.gitlab.gnome.org/gtksourceview/gtksourceview5/porting-guide-4-to-5.html

Thanks,
    smcv

Es gibt eine Familienspende in Höhe von 1.850.000,00 USD von Cheng Charlie
Saephan. Bitte antworten Sie für weitere Informationen. Denken Sie daran,
Ihrer Familie und den Bedürftigen in Ihrer Umgebung Gutes zu tun.

Dies ist bereits der zweite Versuch, Sie zu erreichen. Bitte antworten Sie
für weitere Details.