This is part of the NServiceBus Upgrade Guide from Version 8 to 9, which also includes the following individual upgrade guides for specific components:
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
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.