The notification area in certain Google websites has long hosted a user's YouTube comment threads and Google+ conversations, so Google putting in YouTube upload notifications from subscriptions instead is a natural fit, even if it's only on an experimental and limited basis for the time being. YouTube upload notifications from subscribed channels that you've turned on notifications for normally come to you via Gmail or in YouTube, but some users are reportedly seeing them pop up in the notification drop-down that can be accessed from most Google web pages, as long as you're signed in to Google or Chrome, instead of the normal flow of Google+ notifications and YouTube comment conversations.
The functionality seems to outright replace Google+ notifications and YouTube comments for the users that are seeing the change. The new notification menu presented to these users bears a settings icon; when clicked, it whisks the user away to the notification settings within YouTube, where they can enable or disable notifications for channels that they're subscribed to. There is no indication at this point that the new notification panel will show comment threads, and it leaves out all Google+ business entirely. The new experiment cuts the Google+ clutter to prioritize YouTube notifications, but not everybody will want this change; those that want to roll back can do so simply by clearing their browser's cookies.
This move is only the latest in a string of attempts to separate Google+ and YouTube after the social networking service was tied to YouTube accounts a few years ago. Even with YouTube commenters requiring a Google+ login in order to leave their mark, the community-based service never ended up generating high user numbers or "taking off", so to speak, in any significant way that would differentiate it from the competition and justify users coming from other services or Google sinking significant development time or advertising money into it. Google+ is still functional, and Google has thus far disclosed no plans to change that, but it is gradually being segregated from the rest of Google's services, and this change to the notification area for Chrome users is just the newest step in that long-awaited march.