Agree on the first part but there are going to be plenty of cases where running in a debugger isn't sufficient to satisfy one's self that something never happens. Unless we're talking about "does this happen under this specific scenario I know about".
if something never happens, that's not a part of the codebase you need to understand right now. If your task is cleaning up, sure, but your task here is learning what happens, not what doesn't happen.
And it's relevant in more important ways than "I'm cleaning up and this isn't used, so I should delete it". Quite often, when cleaning up, you find pieces of code that make assumptions that contradict each other. It's a real head scratcher until you figure out that one of these pieces of code is obsolete and never executed anymore (or has never been executed), so its assumptions are irrelevant. The cleanup resulting from that might not look like much, but it removes a major trap when trying to make sense of the code.
