Create 'debian-sys-maint' MariaDB user for use of mysqladmin. Just in case you can't use 'root' via 'unix_socket' plugin.

create_debian-sys-maint_for_mysqladmin.sh

#!/bin/sh
MYSQLADMIN_CFG="/etc/mysql/mariadb.conf.d/90-mysqladmin.cnf"
# generate password
PASS=$(perl -e 'print map{("a".."z","A".."Z",0..9)[int(rand(62))]}(1..16)');
# adjust /etc/mysql/debian.cnf (used as defaults file by system scripts)
sed -i "s/^password =.*$/password = ${PASS}/" /etc/mysql/debian.cnf
sed -i "s/^user     =.*$/user     = debian-sys-maint/" /etc/mysql/debian.cnf
# create config file for mysqladmin itself (maybe not needed)
umask 066
cat > ${MYSQLADMIN_CFG} <<EOF
[mysqladmin]
host     = localhost
user     = debian-sys-maint
password = ${PASS}
socket   = /var/run/mysqld/mysqld.sock 
EOF
umask 022
chown 0:0 ${MYSQLADMIN_CFG}; chmod 0600 ${MYSQLADMIN_CFG}
# update credentials
mysql -u root -p -e "GRANT ALL ON *.* TO 'debian-sys-maint'@'localhost' IDENTIFIED BY '${PASS}' WITH GRANT OPTION;"

Just in case someone lands here - the debian-sys-maint account is not used since 10.4 due to unix sockets, as hinted in this gist's description.

And for this reason, this script is recreating the old behaviour.

Google landed me here, useful thanks!