I’m not 100 percent certain of this but my belief is that:

• Down voting is disabled on a per-instance basis, not a per-community basis.
• When an instance disabled down voting, the button to do so disappears and the display of the number of downvotes disappears when viewing posts/comments as a user of that instance.
• The same post when viewed by a user on another instance will have a downvote button that works, and will see downvotes from their instance and any other that can do so.

So it’s kind of a see-no-evil situation. The downvotes are still out there, but you are unaware of them if if you’re on an instance that doesn’t display them… unless you go check the same post as shown by an instance that displays downvotes.

I’m not sure what happens to a community homed on an instance that doesn’t allow downvotes when viewed from an instance that does. My suspicion is that the instance still handles federated downvotes and just doesn’t display them locally, same as a remote community. But the only instance I know off the top of my head that disables downvotes is beehaw and it doesn’t look like I’m subscribed to anything there to check. Maybe someone else can chime in on that.