I like the idea of it, however, when dealing with several 1,000 bookmarks and lets say a 1,000 are a mixture of 404 or possible 404 its to time consuming to individually select each one so not to include the possible 404.

Maybe group them better (e.g. redirects, timeouts, resource not found, unspecified 40x/50x, server not found, etc.). Allow one to check mark one box scroll down holding shift and click further down and will select everything in that range.

UPDATE 20190911: I decided to start with a fresh profile. In it is a total of 9 bookmarks; 5 of which are the default bookmark preloaded the other are 4 I added. When I run 404 nothing comes back under expired or other which is correct. However, I noticed it reports only 4 unexpired bookmarks rather than 9. So is it intentional to exclude the preloaded bookmarks? NOTE: No Adblockers or any other extension other than 404.