Many (most?) vehicles at least have those defrosting wires in the rear windshield to help improve visibility.
And for most driving, that is fine. People barely use their rear view mirror, let alone their sides. But when you are driving on a mountain road with slush and snow flying everywhere and frost on every surface of your car? You have a massive blind spot behind you (moreso if your side mirrors are frosted or sludged over) and now have to assume there is someone riding your bumper every time you decide how much you have to slow down into a curve and so forth.
I totally was not surprised to see the ioniq5 get rid of them because “Aerodynamics mean your rear window never gets dirty!”. Subaru actually make SUVs for people who do sports (whether you are a lesbian soccer mom or a climber spending a weekend at the crag) and should know better.