Exchange Server 2013 entered extended Support in April 2018. Cumulative Update 21, which came out earlier this month, is the last planned CU release.
Whilst Microsoft may release future CU’s to combat any unforeseen security vulnerabilities, CU21 will be the only update to receive critical updates through Windows Update. Microsoft are urging customers to update to the latest CU so there is no interruption for any potential future updates.
Please note that CU21 requires .NET 4.7.1. You will need to install .NET 4.7.1 before installing CU21. Setup will fail if you are not running the correct version.
Should you require any assistance in upgrading to CU21 or have any queries, please don’t hesitate to get in touch with risual.