gtksourceview3 was superseded in 2018 by gtksourceview4 (a new major
upstream release, but still using GTK 3).

gtksourceview4 has itself been superseded by gtksourceview5 (ITP: #996609),
which is for GTK 4 applications.

Applications that use gtksourceview3 should switch to gtksourceview4.
Even if the application is going to be ported to GTK 4 and gtksourceview5,
it is probably a good idea to move to gtksourceview4 first.

Upstream porting guide:
https://developer-old.gnome.org/gtksourceview/4.2/porting-guide-3-to-4.html

    smcv

It seems the previously linked migration guide is dead, but I found
another one:
https://gnome.pages.gitlab.gnome.org/gtksourceview/gtksourceview5/porting-guide-3-to-4.html