Plusieurs solutions pour l'installation, de pip3 à l'utilisation des dépôts de la distribution. Pour ma part, j'ai installé via les dépôts, sur une Debian 10 Buster :

simon@debian:~$ sudo apt search borgbackup
borgbackup/stable 1.1.9-2 amd64
  deduplicating and compressing backup program
(...)
borgmatic/stable 1.2.11-1 amd64
  automatically create, prune and verify backups with borgbackup
 
simon@debian:~$ sudo apt install borgbackup borgmatic

simon@debian:~$ sudo generate-borgmatic-config 
Generated a sample configuration file at /etc/borgmatic/config.yaml.
 
Please edit the file to suit your needs. The values are just representative.
All fields are optional except where indicated.
 
If you ever need help: https://torsion.org/borgmatic/#issues