MySQL does not support downgrading a database version, and Plesk does not natively support multiple local instances on Linux servers (https://support.plesk.com/hc/en-us/articles/12377060524439-Is-it-possible-to-have-multiple-MySQL-MariaDB-instances-on-Plesk-for-Linux). As an alternative, there are two possible solutions that you can consider:
- You can add an external MySQL server to Plesk by:
- Deploying a MySQL 5.6 Docker Container:
https://support.plesk.com/hc/en-us/articles/12377244574871-How-to-install-upload-a-Docker-image-on-Plesk-server - Deploying an external MySQL DB server and adding it to Plesk manually:
https://docs.plesk.com/en-US/obsidian/administrator-guide/database-servers/adding-and-removing-database-servers.59428/
- Deploying a MySQL 5.6 Docker Container: