Maybe it could just be an alias. People do also refer to them as key-value pairs, so that feels reasonable.
Breaking the API like that would be extremely hostile and exactly the kind of churn byroot thankfully claimed he does not want to produce. If anything both options could be offered. Don't be disrespectful with developers time.
I guess I'm more thinking about Ruby/Rails conventions, e.g. methods like Hash#symbolize_keys.
Mixing the two has always been a smell to me, but maybe you're right.
Sure, but JSON.parse returns a Hash, which is key-value pairs. The method argument is about the return value, not the input value, so it is more like "Parse this JSON, and symbolize the keys in the resulting Hash before returning it to me". Plus, as mentioned, symbolize_keys is more conventional.