For students who were deferred ED1, is a letter of continued interest necessary? On the applicant website, there is already a box to click if you want your application to be reviewed in the regular decision round. I would think the adcom would get annoyed by receiving a bunch of letters providing information they already have.
Is UChicago still your first choice? Why not let them know that? BTW, you needn’t send them ANYTHING they have already and, in fact, they should have conveyed that to you.
In the deferral notice, UChicago states, “If you would like to update us on your recent accomplishments, we would be glad to review additional materials. We prefer that you submit these materials through the Portfolio in your UChicago Account.” This would indicate to me that they would prefer not to hear about your recent accomplishments through a deferral letter.