Released yesterday: https://blogs.technet.microsoft.com/exchange/2017/06/27/released-june-2017-quarterly-exchange-updates/
The blog article and the KB articles do a poor job of explaining everything that has changed. 2016 CU6 includes a fix for the annoying Set-Mailbox bug present in CU5. However, if you do a lot mailbox moves or discovery searches, I suggest that you carefully test it in a lab. The same if you intend to apply CU6 to edge servers. I am not aware of confirmed issues, but I've heard some rumbles about these.
