One of the difficulties in removing a feature is identifying all the potential users. A feature here could be a program bundled with an operating system, or a command line option, or maybe just a function in a library. If we remove a feature, users that depend on it will be sad. Unfortunately, absence of evidence is not evidence of absence. I’ve never heard of anybody running ls -p but it’s not impossible that somebody does.
The reasons why we want to remove an existing feature can vary. Sometimes it’s old code that interferes with maintenance. Sometimes a nearly complete rewrite can improve performance. In other cases, the feature in question is really more of a misfeature. It may have security implications, where the existence of the feature can be used to facilitate the exploitation of other vulnerabilities, and removing the feature will help mitigate the exploit.
There’s no general test that can be used, but there is one test that works in many cases. Test that the feature works. If the feature doesn’t work, that’s compelling evidence that nobody is using it, because nobody can be using it. You don’t need to fix it. You can just remove it.
(If you’ll pardon the heresy, this may be an argument against exhaustive unit tests. Many times a feature will start life in a functional state, but over time falls out of use and then gets broken by subsequent changes. Nobody notices and life goes on. If you have a perfect test suite, you’ll never have broken features, making it harder to identify the unused ones.)