They told us up front the forums would be archived for a time and then removed. We were told to save any posts we wanted. They did that when they announced the forum change over in a post on the old forums. It led to questions about how long the forums would be read-only before being removed. We were not given an answer about it at the time.
The next mention of it was when the comments were removed from News Blogs and we were told it was for GDPR compliance and were told the old forums would also be going away soon. All of them, not just D3. It impacted all games.