Standard my.cnf in our Ubuntu packages has:

!include /etc/mysql/mariadb.cnf

!includedir /etc/mysql/conf.d/

When removing read permissions from the mariadb.cnf file it is just silently ignored, there's not even a warning in the mysqld error output.

When making the conf.d directory unreadable though, by removing the r and x permissions, the server jut outright refuses to start:

mysqld: Can't read dir of '/etc/mysql/conf.d/' (Errcode: 13 "Permission denied")

Fatal error in defaults handling. Program aborted

IMHO an unreadable file should at least raise a warning, and the error handling should be consistent for both "include" and "includedir", failing in the same way if not being able to read the requested file or directory