Free VPN, proxy, Tor and datacenter IP detection. 13 sources, active probing.
IPLogs API is a free Security API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
Free online VPN checker & detection API. Am I using a VPN? Check if any IP is a VPN, proxy, Tor, datacenter or blacklisted. 13-source detection, no signup.
Details from iplogs.com.

Unified API suite · now live
Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
IP network class (residential, mobile, hosting, VPN, residential proxy, Tor) with a confidence score
Reputation of IPs, domains, URLs and file hashes from aggregated threat feeds, plus CVE lookups
The IPLogs API is provided by IPLogs (iplogs.com). Free online VPN checker & detection API. Am I using a VPN? Check if any IP is a VPN, proxy, Tor, datacenter or blacklisted. 13-source detection, no signup.
Yes. The IPLogs API is listed in the Security 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 IPLogs API requires no authentication, so you can start making requests without signing up.
Yes. The IPLogs API supports CORS, so front-end JavaScript can call it directly.
Yes. The IPLogs API is served over HTTPS, so it's safe to use in production.