https://www.techradar.com/news/google-is-planning-to-start-bumping-users-over-to-the-new-gmail

Google is taking its first steps towards phasing out the old Gmail design, replacing it with the modern version that made its debut at the end of April. 

At the moment, the new Gmail is entirely optional. Individual users can choose to activate it themselves, and companies using G Suite (Google's set of apps for businesses) can decide whether to let their employees upgrade, or force them to stick with the classic design.