I can't recall ever seeing this option with the old System Preferences, so it might be new to System Settings; but either way, it's not universal. I have a 5-button Razer mouse attached to test with right now, and the "Mouse buttons" option doesn't appear in System Settings. It does, however, show up in the System Settings search results, which is nice and confusing ("here's a setting we found, that doesn't actually exist for you").