I actually think this is a bug more than a suggestion. I was thinking that it was a problem with the cli importer but now I have tested with the web importer and it's the same. For moderation purposes it's critical to keep the deletion reason for threads/posts. I can't believe you're ignoring this data during the import.
But deletion reason for threads/posts is critical information for moderation purposes. At least to me this is a showstopper to continue with the migration. I really hope you can take this suggestion in the near future. Thanks.