Comment 4 for bug 1740201

gnome-settings-daemon-compat is actually some parts of gnome-settings-daemon 3.6.1, repackaged as "gnome-fallback-[component]-helper". In particular, the background, media keys, and automount plugins were packaged this way. This was, at one time, used by GNOME Fallback, Unity, and other DEs to replace functionality that had gone missing from gnome-settings-daemon.

It has recently become impossible to build a working version of this since GNOME 3.36 introduced a fatal API break (effectively deprecating xorg's built-in background management):

https://gitlab.gnome.org/GNOME/gnome-desktop/-/merge_requests/56#note_705813

What this means for compiz is that it can no longer render a transparent cube in sessions managed (only) by gnome-settings-daemon. Rather, it will render the empty X11 root window as a black surface on the outward-facing sides of the cube (or inward facing sides when using "inside cube"). That's something I'd like to see fixed in compiz, and the goal of this bug report: don't render an empty x11 root window this way; render _nothing_ instead.

What it means for every other DE relying on gnome-settings-daemon is that they have to develop their own, ad-hoc background management. I note that this has already happened in elementary OS, Linux Mint, MATE, etc. A workaround, for example, is to use cinnamon-settings-daemon's background plugin.