Family.categories_last is used for the WOW family because Wikia uses the CategorySelect, which places categories after interwikis. This is only relevant for Wikia sites which have multiple languages, but that is a significant proportion of them.

In core we only have a family file for one case, the WOW family.

Detecting extension CategorySelect removes the *known* need for this Family attribute. However this should be deprecated so that other uses of this attribute have time to complain and find better solutions.