The idea sounds good on the surface but there are some possible issues with this, including potential legal issues.

Subscribers of a channel being shadow-banned could say that its a deceptive practice since subscribing has certain benefits that directly correlate to chat engagement, such as sub emotes and subscriber only chat mode, and technically(?) they aren't getting whats sold to them (maybe) since chat is intended to be a community driven feature where the entire point is to communicate with other chatters and the streamer. I think there are also EU laws that require you to disclose what has happened, a "statement of reason" to the user when they get timed out or banned etc.

Automod already handles this issue pretty well, but often poorly utilized in certain streams. You can flag certain words/phrases that will time people out automatically for varying periods of time (and even auto ban i think).
Yes, its made aware to everyone. As in, you as a viewer or streamer can see when people get timed out or banned (or message deleted) but its relatively inconsequential to the stream if you have a well configured automod.

Additionally, its not hard to tell if you are shadow banned. if you are saying something to get a reaction and don't suffer any consequences you can just look at stream in another browser to look at chat out or ask a friend to see if your messages are visible. and alt accounts are easy to make.

Basically if you implement automod effectively and use features like sub only mode, verified accounts only, and slow mode there isn't much of a reason for a shadow ban feature.

Most large streamers I see that have auto moderation set up poorly and get crazy chatters seem to either not care that much and only ban or timeout due to optics/tos compliance. Or they have a good mod team that takes care of the situation swiftly and dont care about optimizing auto mod.

Smaller streamers often suffer from bad-actors because they dont set-up enough guard-rails, since they apparently don't expect certain spammy or toxic behavior to randomly enter the chat.