Exchange Server 2013 enters the Extended Support phase of product lifecycle on April 10th, 2018. During Extended Support, products receive only updates defined as Critical.
With the transition of Exchange Server 2013 to Extended Support, the quarterly release schedule of cumulative updates will end. The last planned cumulative update for Exchange Server 2013, Cumulative Update 21, will be released in June 2018.
Additional Information:
https://blogs.technet.microsoft.com/exchange/2018/04/10/exchange-server-2013-enters-extended-support-lifecycle-phase/
