Having a @CheckReturnValue annotation always generated would make perfect sense IMHO, because we could then use that in static code analysis to ensure that all users of the RPC do check the Future RpcResult for errors - even if there is no Output.

The "@CheckReturnValue on YANG RPC generated interfaces extending RpcService" thread on https://lists.opendaylight.org/pipermail/mdsal-dev/2017-November/thread.html has the full background.