On a fresh install of Parrot 6.0 sudo apt install picom yielded the following error:
The following packages have unmet dependencies:
picom : Depends: libx11-xcb1 (>= 2:1.8.7) but 2:1.8.4-2+deb12u2 is to be installed
E: Unable to correct problems, you have held broken packages.
It seems that the Parrot repo attempts to install the picom 10.2-3 package which requires libx11-xcb1 2:1.8.7. However, the parrot repo only contains the stable version 2:1.8.4-2+deb12u2. Also, I did not find libx11-xcb1 2:1.8.7 in parrot6-backports.
I fixed this by installing the stable deb package of picom 9.1-1 from the debian repo
wget http://ftp.us.debian.org/debian/pool/main/p/picom/picom_9.1-1_amd64.deb
This works fine for now, but presumably the maintainer will want to know. Otherwise I want to offer my praise to the dev team. 6.0 feels very polished out of the box and I appreciate all of the hard work that goes into this project.