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.
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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.