Comment # 5 on bug 1191362 from Martin Wilck

(In reply to Martin Wilck from comment #4)

> xdg-desktop-portal only activates the openURI interface if
> org.freedesktop.impl.portal.AppChooser is implemented.

... and the implementation of AppChooser was moved from xdg-desktop-portal-gtk
to xdg-desktop-portal-gnome:

* Fri Sep 24 2021 Bj���rn Lie <bjorn.lie@gmail.com>
- Pass --disable-silent-rules, --disable-appchooser,
  - -disable-background, --disable-screencast, --disable-screenshot,
  - -disable-settings, and --disable-wallpaper to configure, make
  use of xdg-desktop-portal-gnome, and more importantly, remove all
  gnome-dependencies from this package.

Thus users (like me) who lack xdg-desktop-portal-gnome don't have the OpenURI()
method. Blame on myself - partially, because I'm using "solver.onlyRequires =
yes"
(but I still hold that that's a must - without it, my package count blows up
beyond reasonable limits).