Hello Wouter,
	 
	Due to some other things in pipeline, this feature has not been scheduled yet , however you can handle this at your own end by sending the expiry time in json. 
	 
	For example: On message receiver event, you can compare the expiry time with current time and  take an action of show/hide notification from the phone. See 
here for ane source project & let me know if it helps.
	 
	Thanks,
	Himanshu Sharma