My paperwhite did an update a month or so ago around the time I started getting emails about this. It’s been nice not converting to .mobi before transferring (sorry Calibre, I don’t need you anymore) but there are still a few glitches reading the format, especially with special characters. Nothing too annoying.
My 4th generation Kindle from 2011 didn't get a software update.
I did a sideload of .mobi today after being rejected by email, no issues.