This is an Office 365 message from Microsoft:
Published On : July 10, 2017
Currently, you are able to return from a list or library to the classic experience using the “Return to classic SharePoint” or "auto-detect automatically", but it is difficult to exit from it. We’re adding a route back from the classic list and library experience in SharePoint Online.
How does this affect me?
You'll be able to choose the new link back, “Exit classic experience." This link will appear in the left navigation area under the navigation links. It will not appear when the list is configured to use the classic mode in the list settings or when the site, site collection, or tenant level opt out. We hope this makes it easier for you to safely update more of your users to the new experience.
We'll be rolling this out in the following days, and the rollout will be completed in the next few months.