• Since Re-prefetch related objects after updating #8043, UpdateModelMixin.update() accesses the queryset's _prefetch_related_lookups attribute.
• Manager objects don't have this attribute.
• This causes an exception when the queryset is, e.g., a RelatedManager:

   def get_queryset(self):
       return self.user.somerelated_set

Notes:

• get_queryset() is not expected to return a QuerySet. This is explicitly stated in get_queryset()'s docstring.
• In fact, that docstring implies that there is no expectation that there is a _prefetch_related_lookups attribute.
• My particular instance of the problem could be fixed on the application side by adding .all(). However, that would cause early evaluation of the queryset before filtering. That's not an option; SQL queries should run only once all WHERE conditions have been applied.

This is a regression that simply breaks stuff that has been working with the stable API. The change should be reverted.

(This is similar to #9306, but not the same.)