Yes, I think they have all become available again. It appears to have been something temporary. That may have been in transition for something big, but we’ll see.

I still have a Kindle 1…although I don’t use it.

For most books (I’m guessing), you are correct about the file being keyed to that specific K1. It’s worth noting that Amazon lets publishers who use their Kindle Direct Publishing to choose not to use DRM (Digital Rights Management). I wish they would put that information on the Amazon product page, but because they don’t, it’s hard to get a sense how many books that is.

Amazon used to explicitly say that you might not be able to download something from the archives again if it was removed from the store for legal reasons.

That always made sense to me. When you download from Amazon’s archives (called the Cloud on a Kindle Fire), a new copy is made. If a book infringed on a copyright, I would think it would be illegal for Amazon to keep copying it and distributing it.

If a book is removed when that isn’t the case, I know from personal experience that you can download it again. I have a book where the author just decided not to have it in the store any more, and removed it. I can still download that.

Recently, I’ve had people telling they could redownload books they believed were removed for legal reasons. It’s possible that changed…but I think it may also be true that the illegality wasn’t sufficiently established.

I think that you losing the ability to redownload something is going to be rare, but you can certainly back up your Kindle if you like. Those file copies will generally only work on the Kindle for which they were downloaded.