I was dev lead for the Microsoft C runtimes (msvcrt.dll and the like) back around the turn of the century. We shipped most of the source for the runtimes with Visual C++, and included a copyright message in each file. Those had the copyright year range in them when I started in the position, but (IIRC) we got the all-clear to dump the years from the text, over 20 years ago. Certainly cut down on source churn at the start of each year.