#2 Oct 21, 2016 15:31
skins_fallback_v5 is needed for all the skins developed with the v5 skin API. They use different layouts for comments, user profiles, etc. If we made the old skins use the new fallback layouts, it would break the old skins.
This will not be dropped for a very long time.
It is likely we'll add
skins_fallback_v8 etc as we add more features / new layouts to the skin API when they are not backwards compatible.