Sometimes when discussing or showing personal things, having a streamer drop in and throw everything on twitch unannounced is kind of off putting, Similarly there are types of content that would get streamers in trouble if they were to show this on their twitch.
The suggestion is to have the ability for the server's owner to mark a server as "please dont stream", while this wont/cant prevent streamers from streaming, this would spawn a warning message to anyone attempting to join on the server saying:
"The owner of this instance has requested to that you do not stream from this world"
[Agree] [Disagree]
(agree would commence the loading of the level, disagree would cancel the joining process)
this would give streamers the ability to either not join or turn off their streams before joining.