Require logging in to view a stream
So in practice this idea seems simple and would be effective. The basic premise of this would be that the Streamer can toggle on/off a feature that would require someone to have a twitch account and be following to view the content otherwise it would just be a black screen that says "Please login and follow to view the following content". I think this could help prevent harassment on various levels as I will describe.
Scenario 1 - Stream Sniping
So for anyone who plays online multiplayer games, stream snipers can ruin a stream quiet fast. Whether it be Fortnite, COD, Fall Guys, etc. there are people who take time out of their day to ruin the fun for others. Now if the feature I described above was toggled on then they wouldn't be able to open the stream in a private window and watch what's going on anonymously and carry on greifing. Instead they'd have log in with an account and be following and therefore at risk of being banned and if they were banned the screen would black out thus not giving them any opportunity. Sure they could make another account however any moderator or streamer on point could pick up on a freshly new account that just started to follow after a ban and act quickly. This would really help put a dent to stream snipers because if they do get caught they can't see the streamers POV and if they can't see it, it becomes much harder for them to ruin the experience.
Scenario 2 - Overall Harassment
This could help deter people who might be intent on harassing someone but can't see the stream unless they login and follow. Thus they are more likely to move on elsewhere or stop all together. For instance someone who is purely making new accounts to harass a streamer would not be able to see the reaction they intend on getting unless they keep making new accounts.
All this could easily work in conjunction with Twitch's Suspicious User Activity that detects possible ban evaders and such thus giving them a black screen until a Moderator reviews the action and either allows them to watch or bans them.
Anyways this is something I have thought about that could stem one of the bigger issue multiplayer streamers face with stream snipers and general harassment as well.
I understand your motivations, but for me it would be ****. Because as a Mod, I often watch streams without not logging in, because I want my peace, and because I saw that more people are using a bot which calls out lurkers in chat. Like greeting ppl who are in the viewer list, but haven't written yet. It is really annoying, and sometimes I want to enjoy a content by keeping my privacy, and without feeling obligated to talk or log in.
Btw, I said "as a mod", because I follow streamers who are in the same circle of those I mod, and if people see me in a channel but not in another, the drama starts.
BUT I would definitely go for it for a banned user. Also wrote this in another suggestion, Banned users not only shouldn't be able to write in chat, but also not being able to watch the stream or see the viewer list.