For what it’s worth, we have also wanted the ability to delete old branch previews.
Knowing that this issue (or “feature”) exists we currently have our netlify.toml set up to pass a build flag that lets us identify branch builds and automatically add a noindex meta tag to the head of all pages, however in any situation where a legacy deploy preview was linked to (or inadvertently indexed by Google) it’d be great if it could be manually pruned via self-service.