Add in /etc/apt/sources.list:

Debian 11 Bullseye (testing):

deb http://apt.jurisic.org/debian/ bullseye main contrib non-free

Debian 10 Buster (stable):

deb http://apt.jurisic.org/debian/ buster main contrib non-free

Debian 9 Stretch (oldstable):

deb http://apt.jurisic.org/debian/ stretch main contrib non-free

Keyring:

wget -q http://apt.jurisic.org/Release.key -O- | apt-key add -

Browse repository :

apt.jurisic.org/debian

Example of installation :

apt-get install nextcloud-server