Since there have been no other network clients besides Derby Network Client tested or supported with Derby since 10.1 and since any protocol based client needs to understand Derby's DRDA extensions, deviations, and stored procedure usage. I think it would be a good idea in 10.9 for Network Server to outright reject any network clients that are not Derby Network Client.
This would eliminate confusion up front for those that might not be aware that the DB2 Universal JDBC Driver and DB2 Runtime Client are not supported. They would get a clean reasonable error instead of hitting various protocol errors.
Also it would mean if someone does want to add support for some network client in the future they would at least need to add the one or two lines of code in AppRequester to identify it, which I think would be a good thing.
I think the code change would not be hard but the biggest impact might be anyone who still runs tests with JCC on trunk would need to disable those tests. There is a separate issue
DERBY-4785 that Jayaram is working on to complete remove the JCC related code from the tests and test infrastructure.