DomainDb Info
Domain name search to find all domains containing particular words/phrases/etc
Domain availability across 1200+ TLDs with registration and renewal prices.
dotsweep API is a free Development API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
Registry-backed domain availability over RDAP with a WHOIS fallback. Prices, minimum term and eligibility included. No key, no account.
Details from dotsweep.com.

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Domain name search to find all domains containing particular words/phrases/etc
DNS records, SPF/DMARC detection and TLS certificate expiry
The dotsweep API is provided by dotsweep (dotsweep.com). Registry-backed domain availability over RDAP with a WHOIS fallback. Prices, minimum term and eligibility included. No key, no account.
Yes. The dotsweep API is listed in the Development category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The dotsweep API requires no authentication, so you can start making requests without signing up.
Yes. The dotsweep API supports CORS, so front-end JavaScript can call it directly.
Yes. The dotsweep API is served over HTTPS, so it's safe to use in production.