After Exchange migration

An Exchange migration was performed with a hybrid config.  

All mailboxes have now been migrated and the onpremise  Exchange is no longer required.

- Must remain as described in various forums for the management of the mailboxes of the Onprem  Exchange?

- If so, the databases can still be deleted?

- Do the connectors  for the hybrid mailflow have to continue to exist or is Azure AD Connect sufficient?