This is part of the NServiceBus Upgrade Guide from Version 7 to 8, which also includes the following individual upgrade guides for specific components:
Starting with NServiceBus.NHibernate version 8.3.0, the built-in gateway API is obsolete and using it will produce the following message:
NHibernate gateway persistence is deprecated. Use the new NServiceBus.Gateway.Sql dedicated package. Will be treated as an error from version 9.0.0. Will be removed in version 10.0.0.
To migrate to the new gateway API: