What happens when a skill reaches zero strenght?
Nothing happens, but I'd suggest strengthening that skill.
Your fluency score will drop. But, nothing consequential will happen.
It stays zero until you strengthen it.
The user script "DuoLingo skill strength viewer" can show you the dead words, that is words with 0.0 strength.
What value is it to know the 'dead' words if there's no direct way to strengthen them?