Not long ago Microsoft have implemented Exchange 2016 within Microsoft Office 365 tenants with future releases enabled. This was seen by a colleague within the company where I am placed today.
After someone forwards his meeting to another customer he notified that the message was sent from Exchange 2016:
This is nice to know, we are waiting when the ECP will be changed to reflect some other Exchange 2016 features within the Office 365 environment.
At this moment the tenant version just show up as Exchange 2015 within the Management Shell: