Hmm, well the last time I did this was to get information for designing Intel Atom into an embedded system- this was ~2019 (I work for a small design firm). I remember you had to sign up for a premier account, sign a CNDA, then ask them to add the specific chipset documentation to your account. Maybe they were forgiving because these were low-end embedded chips.
Anyway, the information for the PCIe registers is in documents like this:
This was the "sign a CNDA" step. They just refused to give one to sign. The specific case was about Thunderbolt controllers, I guess they fact that they're selling licenses to the Thunderbolt trademark might also be a factor. The datasheets themselves have leaked on Mac forums, but that's not enough to build a design on…
…so the alternate option now is ASMedia USB4 chips…
Unfortunately, we are unable to support individuals or smaller entities. Please reach out to your local Intel sales office for assistance.
Are we talking about the same Intel?
(This is the reply I got trying to sign their NDA in order to access datasheets. Even my 15-people employer wasn't worth their time.)