To my knowledge, we don’t do a changelog for the syncs. I believe this is because most of the packages that are updated are not maintained by the Open Robotics team, so it would require a lot of coordination, time, etc.
You can find a list of packages that have changed on the sync announcement (New packages for ROS 2 Humble Hawksbill 2022-09-30). You can then look for the repository and see if they maintain an changelog. Most of our repositories at Open Robotics have a changelog.
I hope this helps!