ETA’s will continue to serve. You won’t be able to edit ETAs, including the URL. If you run into an issue where the destination isn’t working, you should probably remove the ad, or switch to RSA’s. Alternatively, as Marvin points out, you can wait until the URL is working again.
Why we (still) care. If you decide to leave your ETAs on, be sure to keep a close eye on them. It wouldn’t surprise us if more ETAs start getting disapproved in a last-ditch effort by Google to get everyone to switch over.