To follow up here, caribou is the last thing in Debian using clutter. (The forked versions of clutter inside muffin and mutter don't count.) And cinnamon and cinnamon-screensaver are the last things in Debian using caribou.
I filed an upstream bug for cinnamon-screensaver. Cinnamon already stopped using caribou as of https://github.com/linuxmint/cinnamon/commit/5fd5748988a1 but hasn't tagged a release with that commit yet. https://github.com/linuxmint/cinnamon-screensaver/issues/488 Thank you, Jeremy Bícha
