This is easily shown by implementing a TreeModel get_iter vfunc and tracking its return with a weak reference:

import weakref
import gc
from gi.repository import GObject, Gtk

refs = weakref.WeakSet()

class Model(GObject.GObject, Gtk.TreeModel):
    def do_get_iter(self, path):
        it = Gtk.TreeIter()
        it.user_data = 123
        refs.add(it)
        return (True, it)

model = Model()

# This returns a new wrapper and copy of the held struct.
it = model.get_iter(0)
gc.collect()

assert len(refs) == 0, 'a reference should not be held: %s' % refs.pop()