While it creates some work for translators, we should replace GtkStock for the transition to Gtk+3. Reference for how to replace what: https://developer.gnome.org/gtk3/stable/gtk3-Stock-Items.html
This bug was referenced in commit https://git.xfce.org/xfce/xfce4-panel/commit/?id=867275a79b66d521a53e6fcfefdac6af8c5e4b86
Simon Steinbeiss referenced this bugreport in commit 830356520568220f9971f4a9ecf2347a61a0b137 Remove more references to GtkStock (Bug #13528) https://git.xfce.org/xfce/xfce4-panel/commit?id=830356520568220f9971f4a9ecf2347a61a0b137
All meaningful references are removed. The only remaining reference to "stock" is in the GtkCellRendererPixbuf, which is not deprecated: https://developer.gnome.org/gtk3/stable/GtkCellRendererPixbuf.html#GtkCellRendererPixbuf--stock-size