How can we see in github at what moment in time (commit history) a specific release was produced? Is there a way to determine exactly which commits made it into a release and which are still for future releases?
Yes of course it is. Releases have github labels. And are done from branches.
ArduCopter 4.1.x releases will come from the Copter-4.1 branch.
The point in time of the releases can be seen in the Release notes.