Forum Discussion
What is required to decommission the last on-prem Exchange server?
Follow the official guidance here: https://technet.microsoft.com/en-us/library/dn931280(v=exchg.150).aspx?f=255&MSPPError=-2147217396
Pay special attention to the discussion around DirSync - if you decide to keep DirSync, you have to keep the Exchange box for management purposes if you want to stay in supported configuration.
As to what to do with the remaining mailboxes, it's up to you. Just don't "store" them as PSTs... :)
Microsoft announced plans to change this at Ignite 2017. Review Jeff Kizner's session - jump to 54:13 where hybrid recipient management is discussed. I've not heard any further developments yet.
FYI - Jeff Kizner is speaking again at Ignite 2018 and looks like he will provide an update during his session based on the agenda:
"Establishing an Exchange hybrid setup and configuring your tenant are some of the first steps in moving to Exchange Online. This process often gets bogged down in politics, competing priorities with other teams, and general configuration minutia. Fear no more! Learn how you, the messaging admin, can move to Exchange Online without talking to your network admin. Blocked by your pesky security team on the requirements for publishing Exchange? Come learn how you can mitigate their concerns. Come see the changes we’re making to the Exchange hybrid architecture and how we’re simplifying the process of moving to Exchange Online."
https://myignite.microsoft.com/sessions/65633?source=sessions