“Browsers definitely need the certificate for HTTPS, even if you’re just redirecting.
Search engines can probably deal with it, but if there’s a chance the old URL is shown to users, just keep the certificate live too. You can get certificates for free nowadays.”
Mueller hedged his answer about how Google handled this site merge scenario by saying “probably.”
John Mueller later added:
We usually recrawl URLs at latest every half year, so a year gives you min 2 rounds, which is a good starting point. Even without certificate, I'd try to keep the domain name for the long run to prevent spammers from picking it up.