As a BC layer with 4.x, usage of Twig_Loader_String (and later \eZ\Publish\Core\MVC\Symfony\Templating\Twig\LoaderString to fix EZP-22513) was introduced to enable legacy view templates loading into a Twig layout (see TwigContentViewLayoutDecorator).

This currently triggers an E_USER_DEPRECATED error:

DEPRECATED - The eZ\Publish\Core\MVC\Symfony\Templating\Twig\LoaderString class extends Twig_Loader_String that is deprecated since 1.18.1 (to be removed in 2.0)

This is not needed any more in Platform and should thus be moved to LegacyBridge.