In a few weeks, 1% of creators already using YouTube Studio will no longer see the ‘back to Classic’ button – you’ll be notified 1st & can still access Classic features you need! This won’t apply to everyone else until early next year. More info → https://yt.be/help/a7mG
Actually, there seems to be an actual reason they need that change, and if they're able to maybe change the design (which is the main point of the update) it might actually be better than YouTube Classic