This is part of the NServiceBus Upgrade Guide from Version 8 to 9, which also includes the following individual upgrade guides for specific components:
Transports
Other
SendOptions changes
In NServiceBus version 8 and earlier, the indication that a message was marked for immediate dispatch was provided by SendOptions.. In version 9, this method has been renamed to SendOptions..
DataBus interface changes
Starting with NServiceBus version 8, it is mandatory to provide a serializer to the data bus configuration API. The BinaryFormatterDataBusSerializer has been removed and SystemJsonDataBusSerializer is the default option that's built-in.