explain the state of the _thumbnail_id remapping feature

[drzraf]

function remap_featured_images() {}
seems to do that but:

1. is commented
2. depends upon $processed_posts
3. ... which "obsolete" and thus not used

As a consequence, no thumbnail remapping currently happen.
Is it left to "implementers"? Is it just "needs work", and if yes, then where should new code live?