This seems unlikely. IMDB has always been extremely possessive of their dataset. Even in the shutdown notice, they take the effort to explicitly state "for personal use".
Since they had rolling expiry of messages, based on what I've seen, a couple of years of forum posts for less popular movies, and a few months worth for more popular ones.
I'd argue we won't lose anything. IMDB message boards aren't exactly a trustworthy source of information.
Neither is HN for that matter, would you be as indifferent if HN decided to delete all comments?
I wouldn't be indifferent, but also I wouldn't defend keeping them in terms of having value outside of my sentimental affection for what I've written that's been up-voted.
What does being 'trustworthy' have to do with anything in this context?