Here's the issue: There are libraries that have a list of pairs as part of their public API. If we don't add support for migrating this API, each library author will have to do it. I think it would be nicer if we did it. Their code would be immediately simpler. If we have the list support immediately deprecated for some of the functions, I think that would be fine, the libraries' end users would simply need to convert their list of pairs to a map to silence the deprecations.