Might anyone be able to shed some light on what this means for Magento 2 sites using Authorize? I've had weirdly little success finding any discussion on this.
I haven't been able to find much more discussion on it, but others have realized the issue as well: https://github.com/magento/community-features/issues/127
MD5 is getting disabled completely and it means Magento 2 will most likely need an upgrade to remain compatible. Authorize.net has stated that the existing hashes will not be immediately killed when setting the MD5 is removed.