A link back to the source question is only minimally useful 35 months later (and 34 months after it was deleted on SO). Perhaps it should either be demoted to something very small and inconspicuous or removed altogether (without manual intervention) after a certain time has passed:
I totally agree with you. This information is just useful in the first days a post is migrated. After that it isn't useful any more to the general public.
The migration notice is already put in the revision history on both sites (sample here), so it is accessible to every user at any time. I would suggest to remove the notice from the question page after at most a month or so.
Its not necessary for the migration banner to take up so much room, however I'd be hesitant to remove it completely since it is part of the history of the question.
I think it would be useful to have a link somewhere on each question where you could look at meta info about the question itself. This could include migration details, a time-line of voting, edits made to the post, etc.