The seniors have already replied to your query regarding the confirmation letter. Besides the clause in the appointment letter, it is always better to give a confirmation letter to those who are meeting the performance criteria set by the company. Even if an employee does not meet the performance expectations, you can extend their probation by another 3 months. However, the company's expectations should be clearly conveyed to the employee during discussions, and any genuine difficulties or queries regarding performance from employees should be addressed at that time.
The absence of a confirmation letter can lower an employee's motivation to work, and he/she might start looking for another job. So, hand over the confirmation on the due date itself and see the sparkle in the eyes of your employees.
Hope the above clarification helps you!