Thank you for taking the time to report this bug. The problem is that gnome-desktop3 3.34 migrated to Testing before gnome-shell 3.34 did. gnome-desktop3 should have had a Breaks against older versions of gnome-shell.
I suggest you try to install gir1.2-gnomedesktop-3.0, gnome-desktop3-data, and libgnome-desktop-3-17 from Debian Buster to fix this issue until gnome-shell 3.34 migrates to Testing. Of those packages, gir1.2-gnomedesktop-3-0 is the most important one since libgnome-desktop-3-17 is probably still installed on your system. Jeremy
