This is part of the NServiceBus Upgrade Guide from Version 9 to 10, which also includes the following individual upgrade guides for specific components:
Transports
Persistence
Other
Transport Encoding Header
The NServiceBus. header is longer used, but the transport could still send it for compatibility with the legacy Azure Service Bus transport. The ability to send this header has been removed and the SendTransportEncodingHeader APIs to opt in to sending it have been deprecated. Remove all references to these APIs.