It's almost always the
other party in the lawsuit who is subpoenaing your past communications. Your employer is being forced by the court to retrieve these; it isn't their choice. It is in fact possible for your personal communications to get subpoenaed for a lawsuit related to your work. But, the other party will need some evidence that you had a conversation on a personal device that relates to the lawsuit. They can't just subpoena something like "all communications by employees relating to terms X and Y."
Usually your employer would prefer that you not hand over your personal device. Because the other party is looking for some sort of "smoking gun" where employees admit that they think some corporate behavior is bad or breaking the law. The penalties for not cooperating here are bad enough, though, that companies will generally cooperate. And the situations where it's a personal device are relatively rare, usually it's just corporate devices.
Quitting the company doesn't change anything. You can still be subpoenaed after you leave the company and the court can force you to turn things over.
Source: I'm not a lawyer but I have had my communications subpoenaed from my time at both Google and Facebook and I have complained to many corporate lawyers about how stupid these rules are while they politely explained to me how the system works