Since the change adds an upload icon to the bottom bar, the notifications icon has been moved up to the top bar where the upload icon used to be. The subscriptions button will move to the right of the new upload for those in the experiment. The new design is in limited testing for Android users right now, but if the feedback is positive, YouTube says it will consider rolling it out more broadly.
Screens are (relatively) small on mobile devices, so it's important to make the most of the space. Uploading is arguably more important than viewing notifications, and this update puts the app more in line with the web version, which places the notification icon in the top right. I don't think I've ever willingly opened the notifications tab on my phone, so this seems like a change for the better.
A tipster who got in on the test reached out to us with a screenshot of the new upload button in place. As described, it's in the middle of the bottom bar. We've added the screenshot to the article above.