I'm sure there are ways of detecting residential proxy clients that are reasonably effective. I'm equally sure that the people in possession of those techniques (thinking CDN providers, big social networks with public links, newsrooms) are unlikely to share their secret sauce in public. Doing that loses a competitive edge in cost-of-hosting, and as soon as you go public the residential proxy makers can adapt.
There's JA4 fingerprinting (and the JA4+ suite), but still not a surefire way of detecting. Plus the metadata for the signatures has to be constantly maintained and updated.
