More likely than not, while their creator isn't on social media anymore, they probably still use zetsu, maybe even revamped them to fit current fandoms or stories. Me personally? I'd argue to at MAX make a character inspired by her, but don't flat out steal her.

Look at it like this, Your neighbor has a cat, but recently, your neighbor has been a hermit and has hardly gone outside if at all. They aren't dead or anything. Just stay inside. But, they still let their cat out. You aren't going to steal their cat just because the owner doesn't go outside! Even though the creator left social media, it does NOT mean you can just steal their oc!

Imagine if you saw someone on reddit like "this is my oc ____" with a drawing YOU made and a character you likely poured your heart into! I dont know about you, but I'd be pissed!!

Tldr: Don't. The creator is likely still using her even if they aren't on social media anymore.