SharePoint: Enforcing the Classic Experience
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.
Click Advanced settings in the General Settings menu.
When the page loads, scroll to the Library/List experience section towards the bottom of the page, choose the Classic experience radio button then click OK to save your selection. When you return to the document library or list, it will appear in the classic experience with no option for users to temporarily change the view.
Overview
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