What happens to uncompleted sub-tasks when the associated issue is 1) deleted or 2) moved to backlog at the end of a sprint ?
You can't delete an issue with sub-tasks without either moving them to another parent, deleting them, or converting them to issues.
If you move them to another parent, then they become part of that issue and follow that. If you convert them to issues, then they'll drop into the backlog as they're now sprint items, but not ones you've committed to in this sprint.
When an issue is moved to the backlog or the next sprint, the sub-tasks go with it. They are a fragment of the issue, not issues in their own right, they can't do anything else.