Missing gtk4 loop integration

In jupyter console, trying to show a gtk4agg matplotlib plot raises

> UsageError: Invalid GUI request 'gtk4', valid ones are:dict_keys(['inline', 'nbagg', 'webagg', 'notebook', 'ipympl', 'widget', None, 'qt', 'qt5', 'qt6', 'wx', 'tk', 'gtk', 'gtk3', 'osx', 'asyncio'])

(Also, I note that both gtk and gtk3 are deprecated since #1036, is there a reason why? -- gtk (if that means the old gtkagg backend(?)) can probably be removed wholesale as that's Py2-only, but gtk3 should still work.)