If an employee opts out of being contacted by Retain, does it close thier open survey?

No. Opting out doesn't delete or close a survey the employee already has open.

Opting out only stops Retain from sending new invites and reminders. Any survey that's already open stays open, and the employee can still take it from their profile or from their original invite. Opting out doesn't change their profile or the contact info Retain has on file.

Deleting an invitation doesn't close the survey either, nor does it opt the employee out of being contacted. Reminders keep coming, and the survey stays open.

For how an opted-out employee can still take a survey, see Can an employee stop messages but still take the survey?