It's really not that hard to see, on a single team, who's contributing twice as much as who else. You don't even have to be a manager - sometimes managers are the last to be sure, actually. Generally people who complain about lazy coworkers end up settling on a lot of the same people... Being a manager just gives you official venues like feedback requests to realize "oh everyone else sees it too."

Managers are usually terrible at knowing who is contributing on a software team and they're the ones who set pay.

This is 100% a red herring as far as pay transparency is concerned anyway. If you're not being unfair you have no reason to hide anything.