I installed a clean copy of Big Sur on a friend's MacBook Pro (Retina, 15-inch, Mid 2014) a few weeks back. She had installed an OWC SSD. This predates 11.1. Went smoothly. There appear to be issues with the Late 2013 and Early 2014 (Haswell) models of 13" Retina MacBook Pro where Big Sur is causing permanent damage to the right I/O board wherein the Mac won't boot with it connected. That's what this thread is about.
Apple was blocking Big Sur from installing on said models of 13" MacBook Pro, though that was with 11.0.1. I'm not sure if 11.1 is also blocking those Macs.
As for AppleThunderboltNHI.kext, I am unfamiliar with what one needs to do with this KEXT and why. What is it that you do with that file and why? I own that same generation 15" MacBook Pro and have never needed to mess with any Thunderbolt KEXT; though, admittedly, my Thunderbolt usage is minimal.