PyPar2 is Python2 based, so this is why it is missed in modern Ubuntu versions.
It is still installable on Ubuntu 20.04 LTS, 21.04 and 21.10 by manual download of dependencies using commands below:
cd ~/Downloads
wget -c http://archive.ubuntu.com/ubuntu/pool/universe/p/pypar2/pypar2_1.4-7_all.deb
wget -c http://archive.ubuntu.com/ubuntu/pool/main/p/pygtk/python-glade2_2.24.0-4ubuntu1_amd64.deb
wget -c http://archive.ubuntu.com/ubuntu/pool/main/p/pygtk/python-gtk2_2.24.0-4ubuntu1_amd64.deb
wget -c http://archive.ubuntu.com/ubuntu/pool/universe/v/vte/python-vte_0.28.2-5ubuntu3_amd64.deb
wget -c http://archive.ubuntu.com/ubuntu/pool/universe/v/vte/libvte9_0.28.2-5ubuntu3_amd64.deb
wget -c http://archive.ubuntu.com/ubuntu/pool/universe/v/vte/libvte-common_0.28.2-5ubuntu3_all.deb
wget -c http://archive.ubuntu.com/ubuntu/pool/universe/p/pycairo/python-cairo_1.16.2-2ubuntu2_amd64.deb
wget -c http://archive.ubuntu.com/ubuntu/pool/universe/p/pygobject-2/python-gobject-2_2.28.6-14ubuntu1_amd64.deb
[ "$(lsb_release -cs)" == "impish" -o "$(lsb_release -cs)" == "hirsute" ] && sudo apt-get install ./python-cairo_1.16.2-2ubuntu2_amd64.deb ./python-gobject-2_2.28.6-14ubuntu1_amd64.deb
sudo apt-get install ./pypar2_1.4-7_all.deb ./python-glade2_2.24.0-4ubuntu1_amd64.deb ./python-gtk2_2.24.0-4ubuntu1_amd64.deb ./python-vte_0.28.2-5ubuntu3_amd64.deb ./libvte9_0.28.2-5ubuntu3_amd64.deb ./libvte-common_0.28.2-5ubuntu3_all.deb