\Drupal\Core\Plugin\MapperInterface is a general-purpose interface to get plugin 'singletons' based on arbitrary 'options'. It is by no means a proper API, and it's only used by a handful of plugin managers.
$options parameter is so generic, and different plugin managers behave differently based on the array's contents, the interface is useless by definition.
Remove the interface and transfer the method definition to the interfaces of the plugin managers that actually use it. In doing so, we can provide specific documentation for the
$options parameter in the interface of every plugin manager.
User interface changes
\Drupal\Core\Plugin\MapperInterface will be removed. Contrib modules will, if they want to provide a similar feature, have to define a method with the same name in their own plugin managers' interfaces.