SharePoint offers two different user experiences: Classic and Modern. The Modern experience provides a user interface that is highly adaptive to mobile use, and while some may appreciate the look and feel more than the Classic experience, there are some disadvantages to using the Modern experience when managing site components such as document libraries and lists. Additionally, all external SharePoint site components must be in the Classic experience to ensure that the required legal disclaimer is visible to external stakeholders.
It is our recommended best practice, then, to always have external SharePoint sites in the Classic experience.
By default, Sites and Sub-Sites are implemented using the Classic experience and cannot be changed. However, document libraries, lists and other site components can be changed, either temporarily or permanently.
Temporary changes, such as clicking the Return to classic SharePoint at the bottom of the left-hand navigation tab will only remain in effect until you close the browser or exit the site. Seeing this option, or the Exit classic experience option indicates that the site component is not permanently set to the Classic experience.
To enforce a consistent Classic experience on any site component follow the instructions below.
Access the document library or list that is not set to a permanent Classic experience then click the Library (for document libraries) or List (for lists) menu option at the top-left of the page.
Then click either the Library Settings (for document libraries) or List Settings (for lists) option at the top-right of the menu tool bar.