!str #15604: Removal of default fanout

+1, and would be good to (at some point) have a class responsible for this (value class wouldn't weight more).
The same check will pop-up at different places, as here for example: https://github.com/akka/akka/pull/15717/files#diff-5f8030a4c0442271f8985b9ec1f16d8fR47