This is something with the intention for client rather than server side.

So the idea is to subscribe to the streams.online topic but instead of specifying a broadcasterid you specify a userid. This topic would give the streams.online notifications for the broadcasters that the user is following and has notifications enabled for. You could subscribe to streams.online for each broadcaster a user is following which is indeed a possibility but at the same time cause a problem as some people follow a good 200-300 channels of which for quite a lot they probably don't even have notifications which is something we can only check with v5 and probably won't be on helix. Which at the rate v5 is shutting down we would probably be good till 2021 but then we would start to give them notifications for all streamers and would probably need a insane high maxtotalcost to catch up with users that follow a insane amount of channels. Which is why I would like to see a topic that does stuff the other way around perhaps hidden behind a scope or something because internally within twitch there already exists something for it I mean there's gotta be something that's triggering the onsite and mobile notifications and send out emails for the channels people have notifications enabled for.

Possible use case:
Users can sign in with Twitch to a custom minecraft client or use a mod for minecraft or any other game can receive live notifications ingame via the in some games already existing notifications system.