You can test with one user to change the msRTCSIP-DeploymentLocator from SRV: to sipfed.online.lync.com, wait for AAD Sync and check the Interpretedusertype. If this is ok for the user, clear the msRTCSIP-DeploymentLocator and check the Interpretedusertype after AAD Sync.

This is for all users:
Get-ADUser -Filter {UserPrincipalName -eq "UPN"} -Properties * | Set-ADUser -clear msRTCSIP-DeploymentLocator

Here is a good lin, how to remove the SfB onPrem correct with all steps
https://imkarl.me/skype-for-business-server-decommission-after-migrating-to-microsoft-teams/
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