although @techee has a point that as the icons that please Adwaita are used by GTK itself, it's likely not to cause much trouble.
I was looking for this page (which I couldn't find yesterday) for stock icons and their replacements:
https://developer-old.gnome.org/gtk3/stable/gtk3-Stock-Items.html#GTK-STOCK-...
This has been advised by GTK since 3.10 and many applications started using these icons in the meantime so I think we are pretty safe regarding the icons themselves. IMO it's mostly just a matter of using the correct GTK API (plus deprecating some plugin API like `ui_image_menu_item_new()` and having a substitute for it).