Problem/Motivation
The way we migrate comment types from Drupal 6 to Drupal 8 is no longer going to cut it, as we have discovered over in #2853872: Migration for forum and article comments: duplicate comment types and incorrect comment_entity_statistics. That issue is against the D7 migration path, but D6 is affected too because its comment type migration only creates two comment types -- comment and comment_no_subject. This can result in conflicts and problems when the Forum module is introduced, or the Standard profile is installed.
Proposed resolution
The d6_comment_type migration should behave exactly like the d7_comment_type migration and create a new comment type for each node type. To facilitate this, it would be a LOT easier and cleaner to remove the comment type-related source plugins from the D6 migration path, and have the D6 NodeType source plugin include the comment settings for each node type. This will be a million times simpler and cleaner!
Remaining tasks
Write a patch, bikeshed, review, and commit. Not necessarily in that order.
User interface changes
None.
API changes
A few source plugins from Migrate Drupal (which is experimental!) will probably be removed. Therefore, this constitutes a BC break, although it's in an experimental meta-module and therefore within policy.
Data model changes
None.