They are still using backscatter X-ray scanners in parts of Europe, for example Berlin. That airport does allow you to opt out of them.

And in the US, backscatter X-rays have found continued use in jails and prisons. There is actually one that is used in Cook County (Chicago) that is a conveyor belt you stand on to get a full 5 seconds of X-ray exposure. Opting out of the prison scanner may be possible depending on your circumstances, but undoubtedly more difficult