Question: Does phpMyAdmin (web based dbadmin for MySQL) in 3Essential's Plesk control panel support master / foreign keys / linked tables?

Answer:  No, 3Essentials 2.3.0 phpMyAdmin in the Plesk control panel does not implement support for this, however this only means support is not provided in the phpMyAdmin web based admin utility - it does not affect MySQL's support of foreign keys/linked tables when using an database of innodb type.