On the page https://source.android.com/devices/tech/perf/boot-times Google explains it removed CONFIG_MMC from the kernel for the Google Pixel phones. However don't most mobile devices use emmc for flash memory?
Thanks to iFixit I can see now that the Pixel Phone uses UFS instead of eMMC, over the past two years mobile devices are transitioning to UFS I see.
Google Pixel Phone 2016: Samsung KLUBG4G1CE 32 GB Universal Flash Storage (UFS) 2.0
Samsung Galaxy S7: Samsung KLUBG4G1CE 32 GB MLC Universal Flash Storage 2.0
Samsung Galaxy S8: Toshiba THGBF7G9L4LBATR 64 GB UFS (NAND flash + controller)
Nexus 6P: Samsung KLMBG4GEND-B031 32 GB eMMC 5.0 NAND Flash
Nexus 5X: Toshiba THGBMFG7C2LBAIL 16 GB eMMC 5.0 Flash Memory