> should we also implement +/+= for sets?

The question is: what would that do? The same as '|=' ? That would be rather confusing, I think. "|" (meaning: "or") seems a very natural operation for sets, in the same way that "|" operates on bits in integers. That suggests that "|" is the right operator for sets.

In any case, this is an unrelated proposal that is better not discussed in this ticket. The only link is whether "|" is the more appropriate operator also for dicts, which is to be discussed in the PEP and thus also not in this ticket.