Plugins with external_functions that they are not migrated to a new version of Moodle

When you have to migrate Moodle from an older version to a new version, if there are plugins with external_functions in the Moodle old version that they are not installed in the new version of Moodle then, there will be some pages of external services will not view, like admin/webservice/documentation.php.

Example: local_mobile