The above answers are good they deserve the credit I would have updated them with my info here but maybe that is incorrect, I do not want to break their good work.
My use case is a deluged service running with a deluge-gtk watching that service.
I was getting high ram usage due to the ubuntu upgrade 20.4, and the impending upgrade of deluge to 2.0.3+
Also I upgraded libtorrent, that was a nightmare which completely broke everything.
So this 1.3.15 works better for me on Ubuntu 20.4
It also required a lot of removing of lots of applications, but be warned I removed too much and it caused masses of issues.
I was lost due to the various solutions out there on web, that cause all kinds of problems.
If it wasn't for the above answers, I would still be lost.
I followed the above instructions but added deluged / deluge-web / deluge-console
wget http://archive.ubuntu.com/ubuntu/pool/universe/d/deluge/deluge-web_1.3.15-2_all.deb
wget http://archive.ubuntu.com/ubuntu/pool/universe/d/deluge/deluged_1.3.15-2_all.deb
wget http://archive.ubuntu.com/ubuntu/pool/universe/d/deluge/deluge-console_1.3.15-2_all.deb
changes to this command
cat <<EOF | sudo tee /etc/apt/preferences.d/pin-deluge
Package: deluge
Pin: version 1.3.15-2
Pin-Priority: 1337
Package: deluge-common
Pin: version 1.3.15-2
Pin-Priority: 1337
Package: deluge-gtk
Pin: version 1.3.15-2
Pin-Priority: 1337
Package: deluged
Pin: version 1.3.15-2
Pin-Priority: 1337
Package: deluge-console
Pin: version 1.3.15-2
Pin-Priority: 1337
Package: deluge-web
Pin: version 1.3.15-2
Pin-Priority: 1337
Package: libtorrent-rasterbar9
Pin: version 1.1.5-1build1
Pin-Priority: 1337
EOF