Kbin: Not anymore, at least last I checked. I have an old account there that I left behind due to the enormous amount of technical glitches it kept having, and checking in on it recently (maybe last week?), not one of my comments has even a single downvote there - even older ones. iirc the "reduces" tab was still present, just entirely empty. (I was looking for a particular comment, but then while there noticed the effect was much wider.) Edit: I took another look, and I the only downvotes I see are from kbin itself (example post), so it seems to not be federating downvotes from outside of itself.

In the past when it did used to work, it also would not show downvotes from instances that it had server-wise defederated with, although someone can still get downvotes from personally blocking an instance, on a Lemmy server running v0.19.3 or greater, that the server itself had not server-wise defederated with. So there was always a very large gap there.

The reason I thought of this all was due to the OP title: e.g. someone could mass-downvote things on the Fediverse to attempt to control the conversation by de-emphasizing things that they did not personally agree with, but outside of moderator or admin reporting that offers a degree of trust behind it. Obviously that is its intended purpose, but I mean maliciously subverting that like have 10 accounts and log into all of them to influence a post.

About once a week lately I keep blocking some spammer accounts that randomly shill products or videos throughout the Fediverse, rather than wait for an admin to do it, but if an account(s) was more subtle and merely downvoted, then I doubt such a thing would even be noticed?

I should add that I respect some people's decisions if they want to be on a server that doesn't even record or reveal downvotes - that's fine bc it's their choice. But otherwise it is basically public knowledge, except as you say you need to fire up an instance of your own to view them, and then protect that instance from intrusion efforts even if you use it for nothing else (or possibly there is some API call, but I doubt that knowledge would be so easy to find, and for one thing it would have to access a database that has sent out past updates, not merely listen for new ones unless it had been running prior to the downvote event).

Anyway, I hoped people would see this post, and it seems that is happening, so this time the downvotes did not detail any conversation about the topic (with many tens-fold greater up- than down-votes), but if there had been sufficient number of downvotes delivered quickly enough... then how many of us would have even seen this, sorting Subscribed or All by Hot? So it points to a liability in the Fediverse, which at some point, someone somewhere is going to exploit.