So the question is: should librarians be able to enter an issue date in the past?
Effectively, I think they already can, if they upload offline circ. For example, you circulate items for 5 days, but don't upload your offline circ until a week later. The due date should rightly fall in the past. Note that the question was about due date, not issue date, but this example demonstrates that *both* could be in the past.
And unless there is something else "internal" to the logic, this isn't a question for this C4::* function to decide. Validation of the librarian's duedate input must happen at the script level.