https://docs.python.org/3.8/library/collections.html has some examples using collections.Counter, which is clearly described as being a subclass of dict. Amongst the examples:
c + d # add two counters together: c[x] + d[x]
That's the + operator operating on two dicts (don't make me quote the Liskov Substitution Principle), but doing something really different than the base operator.
So if I know that c and d (or worse, that one of them) is a dict, then interpreting c + d becomes much more interesting,
Killing a use of a common operator with a very common built in data type because the operator is used in a different way by a specialized object in the stdlib seems a bit backwards to me.
Frankly, I think considering Counter as a dict subclass is the mistake here, even if it is true.