Well you could just as easily not use Backgrounds and give everyone 2 free proficiencies too, right? I don't know a DM on the planet who doesn't have a list of house rules that alter the base game, so as far as I can tell, everything is optional to some degree.
I mean, you could say you're all non-variant humans, you each get +1 to all your ability scores, and nothing but humans exist. So yeah, I guess Race isn't "optional" in these circumstances, but...it sure feels like it would be.
And what about Alignment? Languages? Experience points? They're not optional, but I know of groups that ignore them.