It is used in social media texting, you say for example : I texted her good morning but she left me on read. It means she read the message but did not respond.

That book wasn't interesting. I left it unread on my nightstand. UNread. Yes, the phrase is used.

No, I don't (and apparently, neither does Lori). As far as I knew, it's an expression used by teenagers who spend too much time on social media :D