For Not Real University, the organization is running in coexistence with Exchange Server 2010, 2013, and 2016.
As with Exchange 2010, you can pause the transport services on an Exchange 2013 Mailbox server to gauge the impact (if any) on your production mail flow.Exchange 20 Mailbox servers can't be uninstalled until they no longer host any mailboxes or public folders.Exchange setup will warn you of any decommission steps that you've missed, and block you from proceeding with the uninstall if anything still needs to be addressed.When Exchange has been removed, you can complete the decommission of the Windows servers themselves by following your standard process.Exchange 2013 Client Access also performs Frontend Transport functionality, so you should also review the protocol logs on the server for any SMTP connections that might still be occurring.
Exchange 2013 Mailbox servers perform the same transport role as Exchange 2010 Hub Transport servers, so you should review them in the same way by using message tracking log searches to determine if any mail flow is still passing through the server.
If you simply shut down servers without cleanly uninstalling them, you'll leave config data in Active Directory for the servers that will cause numerous issues in future.
So you should always take the time to correctly decommission your Exchange servers.
After the legacy Exchange servers have been removed, your migration to Exchange 2016 is complete.
Comprehensive lists of known issues with cumulative update installation generally do not exist, however to improve your awareness of issues experienced by other customers, you should read the comments on the Exchange team blog entry for the relevant cumulative update, and check the Tech Net forums for other reported issues.
When all of the services and data have been migrated to Exchange Server 2016, you can begin to decommission the legacy Exchange servers from your environment.