JTAG is very useful to test that this particular iPhone was manufactured correctly, so Apple doesn’t completely disable it. That said, Apple does have a hardware disable that removes the ability to access registers and memory via JTAG. One only has this ability with special development iPhones that have to be obtained illegally, in the US anyway.

https://googleprojectzero.blogspot.com/2019/10/ktrw-journey-...