I filed now https://salsa.debian.org/mariadb-team/mariadb-server/-/merge_requests/47 as an exploration to fix this issue.
If we don't fix this in 10.11 the alternative would be to patch 10.5 and 10.3 to simply never fail on missing mariadb-client-10.3/5 package. I already did https://salsa.debian.org/mariadb-team/mariadb-10.5/-/merge_requests/14 to make the problem have a proper error message users.
