Need for on-prem Exchange Hub if you're an O365 EOL shop?

Mughal1 If you are running in Hybrid mode then this may indicate that mail is flowing through your exchange server.  If however you have migrated to EOL, then the sole purpose of the exchange server is to give you access to the UI that can update Exchange Objects in AD which are then replicated to Exchange Online.

The guidance on removing the last exchange server changed recently and is published here https://docs.microsoft.com/en-us/exchange/manage-hybrid-exchange-recipients-with-management-tools.  There is a list of pre-conditions that need to be followed.  

The key thing is to SHUT THE SERVER DOWN, but do not uninstall Exchange as this will remove the schema extensions that contain things like Email aliases.

Once this has been removed, you can still use the Exchange Management Tools which will give you the PowerShell capability to continue managing your recipients.

Alternatively, you could consider deploying a tool such as Atria which allows the user lifecycle to be managed through an easy to use portal.  You can find out more at https://www.getatria.com