Whois is now called RDAP
The old Whois is being replaced by the Registration Data Access Protocol (RDAP): structured data straight from the registry of the extension. Since the GDPR, owners' personal data are mostly no longer included, but registrar, dates, status and name servers are.
Some extensions, including .de.eu.at and .ch, offer no public RDAP. There we check via DNS: if the domain has name servers, it is taken; otherwise it is probably available. Only a registration tells for certain.