I know this is late in the cycle to think of this.  But I just realized the PEP 584 does not say anything about order guarantees. 

I think it is safe to assume that existing keys in dictA will not have order modified by `dictA |= dictB`.  However, the PEP does not state what order we are given when new keys are added.  I reckon it is a kind of insertion, so they must occur after the existing keys.  However, stating explicitly that the MUST  (or alternately MIGHT NOT) be inserted in the order that dictB has seems worth specifying.

Moreover, the new operator explicitly states that it will flexibly deal with mapping types other than dict on the right hand side.  So what happens with:

    dictA |= unorderedmapB

I think the "reasonable" belief should be that all new keys will be in some unspecified order amongst themselves, but all occurring after the old keys.  I think a sentence or two explicitly indicating what is promised and what is not would be worthwhile.