Is it possible to use Compiz with GNOME Flashback on Ubuntu 20.04?

The corresponding compiz-gnome package is in place.

So it should work after you install it with

sudo apt-get install compiz-gnome

and optionally set settings using CompizConfig Settings Manager (ccsm):

sudo apt-get install compizconfig-settings-manager

Then select relevant session on login screen.

Yes, Gnome Session Flashback can be used both with metacity and with compiz. In addition to installing gnome-session-flashback, you need to install compiz::

sudo apt-get -y install compiz-plugins compizconfig-settings-manager

Select the "GNOME Flashback (Compiz)" session during login.