Never rely on anything being reliable.
Don’t rely on email as you are at the mercy of Twitch sending them, AND your email provider receiving them
Just poll the API once per minute. Which is the maximum speed you should be polling at.
Partnered streams you could poll for the “xx resubscribed whilst you were away” chat message but thats as real time as polling the API once per minute and means running a bot/chat monitor.
You can either do as @splinny says, or create a dummy account and follow everyone you want. Then poll the /streams/followed endpoint as the following end point returns only the live streams as per https://github.com/justintv/Twitch-API/blob/master/v3_resources/users.md#get-streamsfollowed
Might work out cheaper that constructing a query string argument