Legacy effects are included for project compatibility. These plug-ins are inserted when you load a project (that contains these plug-ins) created with an older Logic Pro version.
You can use these plug-ins or you can replace them with other effect plug-ins available in Logic Pro X.
You cannot directly insert these plug-ins in Logic Pro X unless you override the effects plug-in menu.
Press Option, then click a plug-in slot on a channel strip.
The plug-in menu opens, with a Legacy submenu shown below the Utility plug-in submenu.
Choose the legacy plug-in that you want to insert from the Legacy submenu.