Hello
Firstly, great that you added this new webhook 
One thing which i noticed with it, was that i am only allowed to subscribe to one user at once (i tried adding multipe user_ids similar to the streams endpoint).
I have an application where multiple twitch users can sign up with their twitch account, and then when they come online some actions are executed (e.q. posting automatically on their social media that they are now streaming, and some other things i have planned)
The problem i am having now is that with the webhooks endpoint i have to subscribe for each user which signed up for my application once which ends up in a lot of requests potentially.
Would have been great if it would have been possible to subscribe to online/offline infos in bulk.
At the moment the webhook provides limited help here. Would be great if it would be possible to get users online in bulk? e.q. maybe an endpoint that sends out infos in bulk (e.q. every 5 mins or so that would probably good enough for most usecases) who came online/offline to avoid too much requests on twitches end? which would in turn be useful again to reduce the need to poll unnecessary endpoints