How do I diagnose the last shutdown? adb root? Everything I've read indicates /proc/last_kmsg used to be how, however that seems to have changed with Android 6.0 (I'm on 8.1). I've also seen reference to /sys/fs/pstore/console-ramoops for Android 6.0+, but I have no file called that. I can't ls the directory either, because apparently I don't have root shell with adb.

I've tried adb bugreport, but that doesn't seem to contain shutdown logs.
I've tried adb root, but I get the message:

adbd cannot run as root in production builds


I have a Nexus 5x that is rooted, with TWRP and Magisk.

The only other suggestion I've seen is to edit '/system/build.prop' to include the line ro.debuggable=1, but I can't access that file either due to a lack of root access.

I've spent a week trying to sort this out. Surely it can't be this hard to find shutdown logs?
Why are you trying to read the shutdown logs ? 

May be there's another method of diagnosis.

Have you tried logcat ?