Last week marks the end of support for the legacy synchronization tools which are used to connect on-premises active directory to office 365 and azure AD. Specifically windows azure active directory sync (dirsync) and azure ad sync are the tools which are transitioning out of support at this time. Note also that version 1.0 of Azure Active Directory (AAD Connect) is also transitioning of support. The tools were previously marked as depreciated in April 2016.
The replacement for the older synchronization tools is azure active directory connect 1.1. Customers must have this version of AAD Connect deployed. This is the tool which is being actively maintained, and receives updates and fixes.
Azure AD will no longer accept communications from the unsupported tools as of December 31st 2017.
If you do need to upgrade, the relevant documentation is below: