That would likely be .mobi, which had been deprecated for years. In fact, when I had an Oasis years ago, and I emailed it books I had downloaded, it would refuse them unless they were .epub format.

(Yes, you read that right. Each Kindle has an email address where you can mail your books bought elsewhere.)

I imagine that after several years post .mobi, they are finally done providing that format for older kindles that don't support .epub. It sucks for the people who use the old Kindles, but all e-readers these days, Amazon made or otherwise, prefer .epub as well, so it makes sense that they wouldn't continue to support an old format indefinitely.