In a recent communication, Amazon has alerted Kindle users about significant changes set to take effect from next month. The notification pertains to the phasing out of support for sending MOBI (.mobi, .azw, .prc) files through the “Send to Kindle” feature, starting November 1, 2023. This change, as News18 pointed out, specifically impacts users attempting to send MOBI files via email and Kindle apps on iOS, Android, Windows, and Mac.
Kindles don’t natively “support” Epub, but you can Send to Kindle or even email things to your Kindle and it will get formatted into a format that Amazon will accept. I’ve done this myself for years on Kindles and for devices with Kindle apps.
For your bookmarklet, you’d have to either update it to send as Epub or find another option that sends as Epub instead of Mobi.
In your situation, it sounds like just emailing articles to your Kindle would be the best option. This article can tell you how to figure out your Kindle email and how to send files to it.