I had a previously working Pixel 3XL, running Dirty Unicorns (flashed in December). I hadn't touched the bootloader / fastboot since then, but just a couple of days ago, I rebooted the phone and it refused to boot after that.
The issue is that fastboot cannot determine the locked status of the phone (
Device-State: error!), and therefore behaves as if it's locked (and therefore cannot continue with AVB).
I was curious if something in the system / boot / userspace was able to corrupt the bootloader, or if it was simply a failing battery / emmc. It seems that other people have been struck with this issue, some of them with stock ROM. (I've tried the battery method. It didn't work for me)
(More details in the XDA post linked above. Also any data recovery help would be nice)