As far as I can tell, that only really affects Hexbear users though.

Someone on another federated instance can still see the post from their own instance, downvote it, and potentially see those downvotes there. Hexbear users just don’t have the downvote feature on their own instance.

So, for example, you post something about four-leaf clovers. From Hexbear, you see the upvotes and think, “Cool, I brightened some people’s day.” But someone viewing that same federated post from another instance might see 125 downvotes. They may decide not to read it because of that score, or even add another downvote.

Basically, disabling downvotes on an instance doesn’t necessarily eliminate downvotes across the whole Threadiverse. It mainly changes how voting works and appears for users on that particular instance.

That’s been my experience, anyway. I have an alt account on an instance with downvotes disabled. When I’m logged in there, I can’t see or use downvotes. But when I view the same federated content from my account on another instance, I can see downvotes and even cast them.