When you uninstall LifterLMS all the data generated by the plugin will remain in your database so if you ever re-install LifterLMS or if you are only uninstalling the plugin temporarily all of your students and courses and related data will remain intact.
If you wish to clear you database of all LifterLMS-related data you must add a constant definition to your
wp-config.php file and then deactivate and uninstall LifterLMS.
This was added in LifterLMS 3.13.0. If you’re trying to remove data you should upgrade to the latest version first. Then add the constant and then deactivate and uninstall.