Yes, the Google Smartphones i.e. Pixel and Pixel XL will be having unlockable bootloaders, at least if you buy the device from Google Store.
We already know that Google Pixel and Pixel XL on Verizon will not have unlockable bootloaders, though as per earlier reports both the Google Store and Verizon version of Pixel and Pixel XL will get timely Software updates.
Android.Wonderhowto reached Google regarding this "Unlock Bootloader" thing and Google Spokesperson Iska Saric told them the following:
Yes, Pixel owners who purchased the device from the Google store will be able to root their devices.
Pixel phones purchased from the Google store will ship with an unlockable bootloader.
Unlockable bootloader means one will be able to explore their device in depth by rooting and Installing custom roms and much more.