Twig_NodeInterface is deprecated and will be removed in Twig 2.x but we still use it.
Why this should be an RC target
This is part of making our Twig code compatible with Twig 2.x which was deemed RC eligible in. There should be no disruption involved in making these changes.
Per @effulgentsia in:
I discussed this with the other committers, and yes to removing Drupal core usages of @deprecated Twig 1.x APIs being "rc target" material.
User interface changes
Data model changes