Is it the same for downvotes?

Yes

It's honestly a big downside. I can see it being useful for admins on their own instances to detect brigades or vote manipulation coming from different servers, but I don't like that they are public, because it could discourage people from voting honestly on topics where they could potentially be harassed for engaging

And anyone can be an admin. You can just make an instance and appoint yourself an admin. So in practice it's not limited to anybody.

It's just an option on every post

https://i.imgur.com/j5ZB8vi.png

On the regular Lemmy UI you can see the option if you're an admin (on any instance, not just the one the comment was made on)

No, it's on Lemmy too

It doesn't meet every requirement you listed but privatebin.info supports file attachments, configurable expirey, and burn-on-read, and is fully end to end encrypted

is plasma any good yet? I last used it in like 2019 and it was too buggy. I've heard it's gotten better, but I haven't had the chance to test it

It can be both though.