As discussed in
CASSANDRA-10786, we'd like to add a new flag to the native protocol to allow drivers to connect using a "beta" native protocol version. This would be used for native protocol versions that are still in development and may not have all of the final features. Without the "beta" flag, drivers will be prevented from using the protocol version.
This is primarily useful for driver authors to start work against a new protocol version when the work on that spans multiple releases. Users would not generally be expected to utilize this flag, although it could potentially be used to offer early feedback on new protocol features.
It seems like the STARTUP message body is the best place for the new beta flag. We may also considering adding protocol information to the SUPPORTED message as well.