On this page we have summarised information about downgrading a MariaDB/MySQL database. We do not cover migrating from MySQL to MariaDB or vice versa. For migrations (sidegrade) please look here: MySQL - MariaDB migration and here: Migration between MySQL/Percona Server and MariaDB.
The downgrade process tends to be neglected because nobody really wants to do it. So we are in swampy territory here. Unforeseen situations can occur very easily and good preparation is essential.