I think CalyxOS and GrapheneOS support root ( I know on CalyxOS users were able to root their pixel, so it shouldn't change anything on GOS if I'm not mistaken) but they don't officially support it and refrain from mentioning it.Yeah, I would suspect it wouldn't be able to be rooted since the OS is made to be secure and root is a security issue. I seem to recall that someone might've said about even being able to re-lock the bootloader (and turning OEM Unlocking off after), but not positive.
I haven't found anything on their site about root, but I did see this, which is more evidence that it might not be able to be rooted.
Both COS and GOS have verified boot so the bootloader can be relocked after flashing. But I don't recommend anyone turning off "OEM Unlocking" in Dev Options. Unless you work for the CIA, or you're a dissident etc, travelling etc, leave it enabled (speaking from experiencing! ((not the CIA/dissident part!))
But maybe someone can (and should) correct me, but you won't be able to relock the bootloader after root, which again, should be fine for 90% of users.