I think that's something the XF team needs to investigate and advise on. MariaDB and Mysql up until 5.x/10 have been largely syntactically and file level compatible but that's changing with 8.0/10.3. Going forward I see them only diverging more. I would expect that Maria and Oracle MySQL be both supported in future unless the differences become untenable.
Any version of MariaDB 5.5 or MariaDB 10.x is fine. There may be other MySQL derived products which work fine too. As long as they're based on the codebase of MySQL 5.5 or above then things should be fine.