MyISAM was last supported for Moodle's database in Moodle 2.8 and dropped in 2.9 by MDL-46064. So from Moodle 2.9 onwards Moodle cannot be using MyISAM tables. But there's still:

1. code to check for its presence, for example, mysqli_native_moodle_database->diagnose();
2. language strings, for example, 'myisamproblem' in error.php;
3. A plugin to convert tables: tool_innodb (site note: Moodle had two ways of converting MySQL/MariaDB table engines: tool_innodb, and admin/cli/mysql_engine.php removed by MDL-71476).
4. Moodle Docs pages, for example, Site notifications ("Database tables are using MyISAM database engine - see Migration from MyISAM to InnoDB for advice on what to do").

These are all things that, presumably, cannot occur with any currently supported Moodle version. I suggest this should all be removed.