All that's happening is Amazon is retiring MOBI support. Since Send to Kindle only supported MOBI, they are transitioning Send to Kindle to support receiving ePub which then gets converted to AZW3.
So the only Kindles losing support for the Send to Kindle stuff is the Kindle 1, Kindle 2, and Kindle DX. Since those ones never got AZW3 support.
Anecdotally, I have sideloaded via USB EPUBs and it works on two of my three kindles. Again, I don't know where the OS line in the sand is for EPUB support, but kindles seem to have quietly updated to directly support EPUBs. Amazon could do a much better job of communication here. I assume they aren't communicating this well because they don't want to encourage people to use non-Amazon bookstores.
If this is true you should report it to Mobileread, because this would be a giant story. I have never heard of that being possible, nor has anyone else there. Are you completely sure that you actually sideloaded the ePubs and not AZW3 or KFX files?
I feel somewhat certain I spotted people already doing it on a forum like Mobileread (or maybe it was Mobileread) and that is what encouraged me to try it as I was fiddling with something in Calibre conversion that wasn't working right and searched for "obvious solutions" one of which was a nonchalant "just try to copy the EPUB file over USB". I don't know what to tell you about why it isn't a bigger "story" other than "it seems to be intentionally quiet on Amazon's part" and I guess it is the sort of thing people just kind of do and don't think to talk about?
All that's happening is Amazon is retiring MOBI support. Since Send to Kindle only supported MOBI, they are transitioning Send to Kindle to support receiving ePub which then gets converted to AZW3.
So the only Kindles losing support for the Send to Kindle stuff is the Kindle 1, Kindle 2, and Kindle DX. Since those ones never got AZW3 support.