We are thinking about this and will see if we can find someone in the wider community to build this. There is quite a lot of work in building this in a secure way. Essentially this embed API is expecting Discourse to trust a 3rd party JS payload to run within the same page as Discourse, isolating it into an iframe may be possible but its a tricky thing to build.
I am curious though at a more high level, what specific examples do you use typeform for? Maybe a native Discourse plugin with similar functionality is the path we should take longer term?