Calibre created MOBI files have always worked on the Kindle. That change has nothing to do with the Kindle itself.
The change is for the very specific case using calibre created MOBI file with Amazon's self publishing platform and wanting to have the MOBI file distributed with DRM. calibre created MOBI files previously worked with the service but only when sold though Amazon without DRM. This change was only to allow books to be sold by Amazon with DRM.