Google has taken security pretty seriously on the Pixel and Pixel XL which includes file-based encryption. For consumers, this is great since it makes for a more secure device. However, for developers, it makes things a bit complicated.
For anyone looking to flash ROMs, file-based encryption poses a challenge to installing a custom recovery. However, after some work was done to get around that, some developers have been able to get the popular custom recovery, TWRP, working on both Pixels.