To simplify things, you'll want to start by making sure the DNS entry (
CNAME) for your subdomain is deleted, which basically deletes the subdomain from your domain name. This will ensure that no pages can remain as stragglers, because there will no longer be any server responding at all to requests for your subdomain. This is all you need to do to get your URLs to drop out of search organically.
However, if you want to expedite the process, you could try submitting a Removal in Google Search Console. Using this related post as a guide, you should be able to do the following:
Add and verify your subdomain
https://badsubdomain.example.com in Google Search Console (use the "DNS" verification method, since there is no server there anymore to verify by different means.)
Visit the "Removals" tab, and find the "Temporary Removals" tool.
https://badsubdomain.example.com for the removal request, since you want to remove the whole thing.
Check "Remove all URLs with this prefix".