Is that a setting of the home instance?

Yes. Lemmy instances can disable downvote federation. I will never see a single downvote at the price of not being able to downvote others myself. Which is fine because I either upvote what I consider good content or report content that shouldn't be in the community.

single function and the separate voting system

I don't need downvotes to see the content the community consider good rise above the not so good stuff. It's the same to me if people downvote or don't upvote, both means they're not endorsing it for what ever reason.

So there's a way to separate good content from bad content through upvoting. And there's a way of removing content wrong for the community by reporting it.

So what's left for the downvotes to do? I mean, besides letting angry people do something more than just shout at their screen?