Hi, it depends on the reason for leave. If an employee is getting married and is planning for two weeks of leave, how can an employer restrict it? The same logic applies to sick leave; if an employee is suffering from an infectious sickness, you cannot restrict leave for such cases.
From an employer's point of view, it is true that you cannot hold work or a project just because someone is absent due to reasons like X or Y, as clients will not accept such excuses. Therefore, an Employee Backup Plan should be in place so that even in the event of a key performer resigning, you can ensure a smooth flow of work without any disturbance.