The test I linked is just an example of the technology (it's designed for proxies rather than VPNs, as it just uses websockets vs TCP timing). Tests like http:

The test I linked is just an example of the technology (it's designed for proxies rather than VPNs, as it just uses websockets vs TCP timing). Tests like http://witch.valdikss.org.ru/ http://witch.valdikss.org.ru/ are a bit better, although that particular one has been around for a while and is probably outdated (it flags the WireGuard VPN to my home IP as being OpenVPN, for instance). I don't have Apple Premium so I can't test private relays, but the technology for detecting VPNs is quite sound. It's just not used very often, because there's no real incentive to detect use cases like "people VPN'ing into their home IP". The only exception I've found is maximum-security Cloudflare-protected websites, they seem to run an extra layer of CAPTCHAs when my VPN is on, but those websites are exceedingly rare and probably have some non-determinism in their CAPTCHA ruleset.