@phenaproxima: Regarding most of your comments for s/in/before/: I think it is how we are interpreting those words. I see your point that it might mean that it will be present in Drupal 9.0 but it also means that it will be removed in Drupal 9.0, which means before Drupal 9.0 is released.
I checked how this is used in other parts of the core and it is mixed. Some places mention in while others use before. To me, the meaning is clear with both (because it is semver).
For point 9, I changed it to 'field config and values' as it goes beyond 80 characters.
I am fixing point 12 even though it's slightly out of scope because it is a simple change, but I think points 13-16 can be a follow-up. Actually, code in points 13 and 15 gets changed in #2787639: MigrateCckFieldPluginManager mixes up its behaviour for creating and loading definitions and then we don't need assertInstanceOf at all.