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Possible fix has been provided via XDA-Dev, requires a third party ROM install:

https://www.xda-developers.com/nexus-6p-bootloop-fix/

Gimping your phone just to get it working seems like a terrible "fix"

Another update:

https://androidcommunity.com/nexus-6p-b ... -20170801/

The initial fix disabled the A57 performance cores and was making very poor use of the A53s. The phone would boot but lag and hitch during routine use.

In this patch, the A57s are still disabled, but now the A53s are fully utilized and a formerly bricked phone is, according to the dev, usable as a daily.

Same kind of crap would happen with my Samsung phones on "official" AT&T releases that would come out about a year after the phone was done being paid for. On two phones.
This Samsung SII was a bug that ate battery in about 4 hours when the phone wasn't in use (It would get hot as hell in the pocket). It took AT&T almost 6 months to release a fix, but by then I had used 3rd party ROM to make it work. In the meantime it ruined the battery.

Then the next Samsung phone started to "pretend" the SIM card was undetectable. Sure the socket was poor, but there was no excuse for not giving it a retry upon an error and locking me out of the phone. Of course, no warranty on that either. 3rd party ROM fixed that also, but I got fed up with goofy sacrifices to make it work like disabled features. My son is using it to satiate his gaming addictions with no phone functionality.

Now I am using a Motorola Droid Turbo II, and love the phone, and on Verizon. No issues at all.

All I can figure is Samsung hardware has quirks that are difficult to write software around, or AT&T is out to push people to upgrade phones by planned obsolescence and make more money.

Lo and behold...it just happened to my own Nexus 5X tonight. I had been having some unexpected issues (including some problems accessing the phone from a PC over USB) but didn't realize what was up. Tonight, it suddenly locked up while browsing through the UI and it would not boot after that. I popped it open and briefly disconnect the battery. After that it would power up again, but never complete a boot cycle. Fortunately, this also meant it could be booted to Recovery Mode.

After some wrangling (which included freezing the phone - literally, as in get the unit down to abou 40F) to briefly get around the boot loop so I could access Developer Options and enable OEM Unlocking, and also installing ADB+Fastboot on my PC, I was able to flash 4CoreOreo.img onto my v8.0.0 phone and reboot. One factory reset later and it looks like we're sort of back in business, although it's still prone to lockups or spontaneous reboots if it gets warm, and it definitely hitches a lot more than usual.

Not ideal but at least I can get into the phone.

EDIT: I ordered a new Pixel, case, and screen protector on Amazon and paid through the nose for 2-day shipping...should have it before the Thanksgiving holiday travels. No way can I rely on this 5X to coordinate with my wife through airports. Goodbye, $600

This mess has really put me off getting another LG phone. I already had to replace my motherboard a year after I got the phone, and I'm dreading the day when this replacement also bites it. I really don't want to go down to A53 cores, they suck donkey dick.