WordPress usage stats: https://wordpress.org/about/stats/
Technically, 10.x versions are MariaDB
WordPress requirements: PHP 5.6.20+ and MySQL 5.0+
WordPress recommendations: MySQL 5.7+ or MariaDB 10.2+
Mysql: release history, downloads: stable, archive
MariaDB: release history 1, 2, downloads: stable
changelogs: 10.6
Compatibility: https://mariadb.com/kb/en/mariadb-vs-mysql-compatibility/
Differences I met
MariaDB doesn’t support ->> -> operators (issue)
Use their regular equivalents JSON_EXTRACT/UNQUOTE (docs) instead.
About JSON in MariaDB: https://mariadb.com/kb/en/json-data-type/
MariaDB syntax of generated columns is a bit different (it doesn’t have NULL / NOT NULL part).
Docs: mysql, mariadb