The YouTube Gaming app was officially shuttered at the end of May, and with it... well, not too much was lost. The app itself was basically a heavily skinned alternative to the regular YouTube app, but with a strict focus that limited video suggestions to game-oriented content. While most things were moved to the main YouTube app a while ago, one core feature didn't make it over before YouTube pulled the plug: Screencasting. That oversight has been corrected in the latest update to the YouTube app, so you can livestream your gaming sessions (or whatever else) like before.
Screencasting support was not unexpected, the first signs YouTube had started work in migrating it to the main app appeared all the way back in February with the release of v14.08.
Unfortunately, there are some differences from the original implementation in the dedicated Gaming app that might leave many users hanging. First, this is only for screencasting. The ability to make a local recording, which could be viewed or edited later, is not present. To make recordings, you'll want to use the Google Play Games app instead. The other catch is that users with fewer than 1000 subscribers on their channel are not permitted to do mobile livestreams at all, regardless of whether it's a screencast or using their regular camera. They can still go live from a regular computer. (This was the one detail that didn't change in the 2017 policy update.)
If you would like to do a live screencast to YouTube (and have the requisite 1000 subscribers #salt), just hit the video camera icon at the top of the YouTube title bar and hit the button to start a live stream. Tap the little mobile icon appearing at the top of the setup page (first screenshot) and continue by picking a game to stream and the screen orientation you're going to use. At the end, you'll also get a couple reminders that you should be careful not to reveal any personal information.
Once the settings are in, you'll have the traditional selfie cam bubble, which can be dragged around the screen, and a control panel with buttons to toggle the selfie video, the microphone, and visible comments. There's also a settings button that opens a slider that can change the size of your selfie bubble.
Now, go forth and make that gaming content; YouTube needs your help to catch up to Twitch, or maybe Mixer.
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