Squash 1.21 is to be released late December 2019. For this 1.21 version and future versions, Squash support will officially cover two database management systems: PostgreSQL and Maria DB.
For MySQL users, if you choose to upgrade Squash 1.21, MySQL will not be supported anymore by Squash Support.
Following the acquisition of MySQL by Sun Microsystems and Oracle, a MySQL community fork has been created: MariaDB. The MariaDB Foundation manages the MariaDB project.This will ensure that the Database management systems remain free.
You will be able to upgrade from MySQL to MariaDB by consulting the documentation here.
For the sake of completeness, if you use 1.20 Squash version or older versions, MySQL will be supported by Squash team.