We will eventually need to migrate Drupal 8 to Drupal 9 (and possibly Drupal 10).
Also while it's less common, there are use-cases for migrating Drupal 8 to Drupal 8 - things like migrating a multi-site to domain module.
Add Drupal 8 source plugins, and test coverage for an 8-8 migration.
This will provide the basis for the 8-9 migration path, since these would only change to due to changes in the migrate API in 9.x
If we add this before opening 9.x then we have the potential to keep 9.x in a 100% shippable state with support for 6/7/8-9 migration paths (even if we might move the 6.x sources to contrib at that point).
This is a much lower priority than 6.x/7.x sources, so marking postponed.