gtksourceview3 was superseded in 2018 by gtksourceview4 (a new major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.

This particular package uses gtksourceview3 indirectly via the C++ binding
libgtksourceviewmm, which seems to have become unmaintained upstream (see
#1079433). If it is desirable to keep this package in Debian, then someone
with an interest in gtksourceview C++ bindings should take over upstream
maintenance of libgtksourceviewmm and finish releasing a 4.x version.

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.