Earlier quoted context omitted.
Most of these features can be employed on the website side perfectly fine. Tools like https://obfusgated.com/tools/vpn-detection-test will use very basic timing tricks to detect VPNs, for instance. Not very reliable, but if you add data points about things like MTU size and WebRTC, and cross-reference that with GeoIP location, you could detect most VPNs. Not without rejecting a small amount of edge cases (Firefox use…
BTW, I connected to that test using Apple’s Private Relay and it rated my VPN-ness as 0%. I stand by my point.
Tests like 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are a bit better, although that particular one has been around for a while and is probably outdated (it flags the WireGuard VPN to my home IP as being OpenVPN, for instance).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.