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You are currently looking at documentation for Umbraco 8 and older versions.
Go to docs.umbraco.com/umbraco-cms/reference/configuration/contentsettings#error-404-collection for documentation for Umbraco 9 and newer versions.
How to implement a 404 Page
To implement your own 404 finder, create a class which implements the interface IContentLastChanceFinder and registering it as the last chance content finder using an IComposer. A ContentLastChanceFinder will always return a 404 status code. - Using an IContentFinder for Custom 404s