Tesla definitely added a third row of seats in the Model Y with the clear idea that only children or people who are not tall at all can travel back there. Sure, it's nice that a relatively compact and coupe-like crossover like the Model Y even has a third row, but if taller passengers would ever be there when the car was in an accident, chances are they'd be subjected to massive head trauma.
You've probably seen videos showing just how little room for your head there is in the rear of a Model Y - most people's heads touch the glass on the hatch, yet if they move their head forward, they will hit the hatch itself. Now imagine that in the event of a frontal collision, your head would hit that ridge causing massive injuries.
In the event of a rear end collision, taller passengers' heads would probably break the rear glass and then hit the rear edge of where the glass used to be. None of these scenarios are particularly rosy and the main problem is Tesla chose a fastback-style rear end that impedes on headroom.
Had the Model Y been a more conventional crossover, with a more straight roofline that allowed for actual headroom in the back, the third row would have been a safe place for adults. As things stand, if you are an adult of at least moderate height, then you will be extremely vulnerable (particularly to head injuries) if you are caught sitting there in an accident. Kids or anybody whose height allows their head to clear the roof and not hit it should be fine, though.