I wonder what percentage of kindle users even knew about this or used it.
AFAIK this was pretty integral to the Calibre + DRM cracking plugin workflow that everyone used to buy Kindle books and then use them on non-Kindle platforms. In other words, it's Amazon welding shut another unauthorized fire exit.
I don't know if that matters to them, or if it even registers tbh, but I think I'm like most people in that regard: I'm not going to struggle to send feedback to a megacorp, I'm just gone.