You are currently looking at documentation for Umbraco 8 and older versions.
An automated guess is that docs.umbraco.com/umbraco-cms/reference/configuration-for-umbraco-7-and-8/404handlers/ could be the link to the new documentation for Umbraco 9 and newer versions.
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