How can I solve a "Version Error" when trying to run rdiff-backup between the latest Ubuntu release and the latest Security Onion release?

Background information / previous post available here: Need to try to install a specific package version -- however apt-cache madison, apt-cache policy only seem to list the latest version, though I don't think it'll be particularly necessary or helpful to read.

I was trying to run rdiff-backup, which failed as shown here. The issue seems to be that Security Onion (I'm told) runs on an older version of Ubuntu, limiting it's rdiff-backup version / repo available to 1.2.8, whereas the latest version of Ubuntu is running 2.0.0 (if I recall the exact version number correctly, which I think I do.)

Now I pretty much got the answers I was hoping for as far as playing with apt-get install and being able to download a specific version -- but somebody recommended I post this as it's own question, so I wanted to follow suit in case it yielded any helpful answers.

As of now, my workaround is just to install an older Ubuntu version; since for my strictly homework purposes, it doesn't matter what OS I rdiff-backup to, just that I backup to a different box with it.

In case the remote-server/destination-server is running an old version 1.2.8* and the local-server/source-server in running the new 2.0* version rdiff-backup will not work.

...
version of rdiff-backup is quite different from the local version (2.0.0).

In order to solve this issue with the differences, both the (source-server) and the (destination-server) must be in the same major versions (1.2.x).

The solution is: (tested and worked fine for me)

sudo su 
echo 'deb http://cz.archive.ubuntu.com/ubuntu bionic main universe' > /etc/apt/sources.list.d/rdiff-backup-1-2-8-pkgs-ubuntu.list

apt install -y librsync1=0.9.7-10build1 rdiff-backup=1.2.8-7

Confirm the installation version :

rdiff-backup --version

With that source and destination servers with the same version the cms for using rdiff-backups should be fine:

rdiff-backup /PATH backups-server::/path