It’s finally happened.
Amazon has announced that soon it will no longer accept* Mobi files in KDP. They just sent out an email informing authors and publishers that Epub and Doc are now the preferred formats:
We listened to your feedback and are making it simpler to publish eBooks on Kindle. Starting June 28, 2021, we will no longer support files in MOBI, PRC or AZK formats when publishing new reflowable eBooks or updating the content for previously published eBooks. Instead, we ask publishers to use EPUB, KPF (Kindle Create files), or DOC/DOCX (Microsoft Word files) files for reflowable eBooks. Please note MOBI files are still accepted for fixed-layout eBooks.You don’t need to take any action for reflowable eBooks already published unless you are updating the eBook files. If you are uploading new or updated content, see our Kindle Publishing Guidelines: https://kdp.amazon.com/help/topic/G79CTKR8BX79E96L
And before you nitpick my language, yes, I know that it’s clunky. So is their new rule, which is basically that Amazon will only accept Mobi files for the one type of Kindle ebook which no one makes any more.
Why they decided to keep that around, I do not know.