I am not going to say it never happens but purposely induced delays are very rare. You have to remember that Twitch is a community and the people watching communicate with the streamer through chat. Getting a response to something you say in chat 5 minutes later is not a workable solution.

Now, this works fine for large broadcasts where there is no two way communication between a streamer and their followers or in some type of competitive situations, but that is not the norm.

All the streamers I watch respond to chat in digest form after finishing a game

Twitch is just one example, and I'd assume most live video viewed would benefit from a lower delay. That goes for everything from security cameras to sports to video chat. I'd bet that for the vast majority of live video a lower delay would not be a "liability", but rather a plus.

Iter-frame parallelizability is made possible (at least in the context of h264) by using slices. It comes with a minor penalty in the resulting data compression ratio.