RDAP (Registration Data Access Protocol) is the modern successor to WHOIS for querying domain and IP registration data. Where WHOIS returns inconsistent free text, RDAP returns structured JSON over HTTPS, making it far easier to parse programmatically.
For takedowns, RDAP is how you find the registrar and its abuse contact for an impersonating domain — query https://rdap.org/domain/example.com and read the registrar entity and the entity with the abuse role. RDAP returns the registrar and abuse address even when registrant details are redacted under privacy rules.
RDAP is gradually replacing WHOIS as the standard registration-data service. See the guide on reporting a fake domain to a registrar.