Recently, I have been digging more into how to discern if a lossless file is truly lossless and not an upsampling/upscaling of a lossy source. It turns out that looking at the cutoff in the frequency spectrum is not sufficient to truly judge the native bitrate. However, there's a software that goes through more sophisticated tests/algorithms to check for upsampling/upscaling: http://losslessaudiochecker.com/
Using this software I checked the most recently leaked lossless instrumental albums. Here's the result ['clean' meaning no upsampling/upscaling detected]:
Hybrid Theory - all clean
Reanimation - all clean
A Thousand Suns - 'Wisdom, Justice, and Love', 'Fallout', and 'The Messenger' upsampled, others clean
One More Light - 'One More Light' upsampled, others clean
Post Traumatic - 'Place To Start' and 'Nothing Makes Sense Anymore' upsampled, others clean
I am not entirely sure if we can trust the evaluation a 100% but it should be better than just looking at the frequency spectrum. My guess is that the songs that were detected to be upsampled were taken from another lossless source (Post Traumtic EP, ATS album, OML multis) and slightly modified to match the other files in terms of bitrate.