A cloud-based identity and access management service for securing user authentication and resource access
The group-based scoping should only be used at the initial, testing stage of a deployment. In your scenario, use either OU-based or attribute-based filtering, as detailed here: https://learn.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-sync-configure-filtering
And yes, users that are already replicated to Azure AD and are removed from the sync scope will be deleted, so thread carefully.