Legacy mode in Joomla can be best defined as a mode which when turned on (enabled) allows all the old version compatible modules, components, and Plugin of Joomla to run successfully in the current or latest version of Joomla.
Joomla 1.5 comes with a default Plugin named ‘System – Legacy’ .As soon as we enable this Plugin from the Plugin Manager, the legacy mode gets activated in Joomla.