Is it okay to use MySQL Workbench with MariaDB?
This warning is shown for any other but the supported MySQL versions (at the time being these are 5.6, 5.7 and 8.0). The application uses language features and other resources that may or may not work with other servers. It's actually not tested, which is the main reason why this warning comes up.

MariaDB might be based on MySQL, but over time it more and more diverts from its root and it's very likely that there is an increasing number of incompatibilities.