On the other hand update() method may accept multiple iterables at once:

  def update(self, *iterables):
      for it in iterables:
          self |= it

and therefore it is not equivalent to __ior__ method. In this case:
'difference', 'intersection', 'union' set methods could also be added to
Set and 'difference_update', 'intersection_update', 'update' to
MutableSet.

Negative consequences:

- no use-case?

- there are more than one way to spell an operation e.g., &= and
  intersection_update for a single iterable case

- documentation, tests, methods implementation have to be maintained in
  sync with frozenset/set

Positive:

- Set/MutableSet can be a drop in replacement for frozenset/set without
  manually reimplementing the named methods even if multiple iterables
  are not used

- documentation, tests, methods implementation are maintained only in
  stdlib and therefore bugs are fixed in a single place and the same
  behavior everywhere

I've uploaded a prototype patch that implements the named methods,
mentions them in Set/MutableSet documentation, and adds sanity tests.

If somebody provides a compelling use-case for adding the named methods
then further work on the patch could be done.