If ragequitting, why bother with making a new release instead of just removing _everything_ and changing the webpage? (Presumably, you already have a copy of the SW if you have encrypted volumes.)
Also, note "you _should_ migrate data". Could this imply that cold storage is not secure?
There are any number of possible vulnerabilities that could exist. Its definitely a plausible possibility. I could fairly easily believe that they were contacted by a researcher who was about to publish a major AES flaw, or one of the other algorithms in use.
There's a number of relatively plausible theories. I wouldn't be surprised if we don't find out for 20 years what the actual reason for this was, when the developer is on their deathbed.