This is from Pearson / InformIT in an email I just received. Pertinent sections follow. Any errors are theirs.
"You are receiving this message because you have purchased one or more multi-format eBook titles from one of the five InformIT Network websites, and those files have been made available to download from your account page in three formats: EPUB, PDF, and MOBI. The MOBI format will no longer be available for download as of August 18, 2022.
"Amazon has announced the retirement of its proprietary MOBI file format for eBooks, and a transition to the EPUB format for use on Kindle devices and apps. As of August 2022, you will no longer be able to send the MOBI file format to Kindles or the Kindle app using the Send to Kindle service. The MOBI file format no longer supports the newest document features, while the EPUB format is updated and accepted by Kindle devices.
"Any MOBI files you have already sent to your Kindle will not be affected by this change. However, the downloadable multi-format eBook files available on your InformIT Network account will no longer include the MOBI format; those links will be removed on August 18, 2022. The EPUB and PDF links will remain. Purchases of future multi-format eBook files on any InformIT Network site after August 18 will include the EPUB and PDF formats, but will no longer include MOBI."
@craigmaloney hmm, does this mean a slightly old kindle that doesn't do epubs will start to become a bit useless?