Decommission your on-premises Exchange servers after using hybrid migration

 As of writing (07/09/2020)  the short answer is:

1. Don't, at least not fully if you are keeping AD FS or AD Sync.

For the long answer see (https://docs.microsoft.com/en-us/exchange/decommission-on-premises-exchange) and the following blog (https://techcommunity.microsoft.com/t5/exchange-team-blog/decommissioning-your-exchange-2010-servers-in-a-hybrid/ba-p/597185)

But basically if you fully remove the Exchange system while still keeping AD Sync which is a requirement for AD FS too then your user admin tasks for Exchange 365 becomes more tricky as you will have no EMC on-perm but all the Exchange settings are still stored there and not be done via Microsoft 365.