It's possible, yes, though not likely, unfortunately. The Swift 5.1 window is still open for overlay changes as far as I'm aware. My concerns about compatibility voiced in the PR have not changed, but if we go with @norio_nomura's PR, the new API would have to go through API review (since this is Foundation API, it would have to go through Foundation API review, not Swift Evolution, and I don't know if we have the bandwidth right now).
[I also forgot I had started a thread up about this a while back in Handling edge cases in JSONEncoder/JSONDecoder key conversion strategies (SR-6629)]
/CC @bendjones for visibility