MODULES-2676 added support for MySQL 5.6.7+ by introducing changes such as as a conditional in mysq_datadir to remove the dependency on mysql_install_db. Since mysql_install_db is no longer installed and used in MySQL 8.0, the mysql daemon will provision the database. However we still have a hard requirement on mysql_install_db as follows.
The requirement on mysql_install_db is a blocker for MySQL 8.0 as this command does not exist on the system. The mysql_install_db command should be optional to ensure future compatibility.