That also means that when ATC tells someone they will inform them in 10 minutes,they are obliged to tell them in 10 minutes, even if it's "sorry, we don't have

That also means that when ATC tells someone they will inform them in 10 minutes,they are obliged to tell them in 10 minutes, even if it's "sorry, we don't have a slot, we can get you info in next ten minutes or help you to diversion airport". Not have to be reminded that there's a plane in holding waiting for information. Unless they want a repeat of telling a plane to hold till it crashed into sea.