cat /etc/apt/sources.list.d/multimedia.list
# multimedia
deb http://www.deb-multimedia.org buster main non-free
cat /etc/apt/apt.conf.d/99enable-unsecure-repos
# Acquire::Check-Valid-Until "false";
Acquire::AllowInsecureRepositories "true";
# Acquire::AllowDowngradeToInsecureRepositories "true";
apt-get update
apt-get install -t deb-multimedia mpv
Reading package lists... Done
E: The value 'deb-multimedia' is invalid for APT::Default-Release as such a release is not available in the sources