I think the problem here is that this is a Gmail-specific, non-standard feature, meaning there is no specification that allows to implement this in a reliable, cross-client way.
However, Fastmail is working on something similar with JMAP.
> However, Fastmail is working on something similar with JMAP.
They are! [1]
"It really is coming in JMAP! Come along to IETF in London to have a play, or I'll be pushing out links to updated JMAP Proxy in the next couple of weeks and you can play with it there :)"
Once they've got JMAP nailed down, they'd just need to support Gmail's native API [2] for retrieving rich message data for their import process.
I think the problem here is that this is a Gmail-specific, non-standard feature, meaning there is no specification that allows to implement this in a reliable, cross-client way.
However, Fastmail is working on something similar with JMAP.
I found this earlier HN comment to be really interesting: https://news.ycombinator.com/item?id=16372835