Kinda. I just helped my girlfriend setup a new Windows laptop the other day, and the usual DeDRM tool no longer worked, because the books were downloaded under the most recent version of Kindle for PC, which uses some new form of encryption that hasn't been cracked yet. We had to uninstall it, download an older-but-still-supported version of Kindle for PC, then run through the process again.

Which is to say, this is a cat-and-mouse game. There is a way to defeat Amazon's encryption for now, but there may come a day when we can't, and when Amazon cuts off support for all devices and applications that do not run their latest encryption.

I'm kinda surprised that they continue to even support MOBI, given that it's not originally their format and much easier to extract the EPUB out of than their homespun solution.

You need to download the files from "Manage Your Content and Devices","Actions", "Download & transfer via USB". This gives you a different format, and DeDRM can unlock these.