How does this affect me?
SharePoint sites with the publishing feature enabled currently load the classic UI for all document libraries and the Site Contents page in that site, except for document libraries that have explicitly enabled the modern experience in library advanced settings. We are changing it so that sites with the Publishing feature enabled will load the modern UI for site contents and documents libraries, unless an administrator has opted out of the modern experience.

What do I need to do to prepare for this change?
If you want certain sites with Publishing enabled to continue to use the classic experience, you can configure them to do so. For information on how to do this, click the Additional Information link.