Note that some WHOIS servers (as in port 43) are hostile to automation and might block you if you’re doing it too much, although you could be paying a few cent per custom domain per month to have a service perform the data aggregation for you.
Agree, most ToS of WHOIS servers say they don't want automated queries.
In this case I think it'd be simpler to just do a DNS check, either NS alone or A/MX. Scales better. If NXDOMAIN is returned over a period of days then the domain can be flagged.
