In #2887142: NodeType source plugin should include comment information → Comment variable sources were deprecated, and a layer of code was added to the comment source code as a bc layer for migrations using these old sources. This BC method can be deprecated in Drupal 9 and have it's usages removed in Drupal 10.
It was un-deprecated in #3069055: Undeprecate Drupal\comment\Plugin\migrate\source\d6\Comment::prepareComment() →
Remove the usage and deprecate the bc layer method for removal in Drupal 10.
Do it! just not yet.
Fixed
9.3
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.