It is a fairly standard practice in at least some open source communities to add copyright notices to files that people have changed significantly, although there is no well defined minimum threshold for how much permits them to add a copyright notice. Thus, someone else can come along, fork the project, add copyright notices to all of the files and then give the impression that they wrote them, since there is no attribution aside from the one LICENSE file that you wrote. The git history might show the truth, but if they copy the files into a fresh git repository, that metadata will be lost. Projects take files from one another all the time, so there is no guarantee that they will preserve your commit history and then anyone curious who wrote the code needs to do digital archaeology.

That said, file level copyright notices are not perfect (since only the VCS shows who added what lines and that might not be preserved), but it is better than nothing and it is something that is guaranteed to persist as long as people are abiding by licenses. If they are not, that is copyright infringement and the copyright holder can do things like send cease and desist notices in response to the copyright notices being removed.

Also, I must emphasize that I am not a lawyer, but one might argue that it was not willful infringement if someone removed a copyright notice from 1 file by claiming it had been a mistake. However, if they remove it from all files, then nobody is going to believe it was not willful.

Sufficient but a good idea to put copyright in all files.

Technically if there's no license found then it should be considered automatically copyrighted, with no permissions to copy. So leaving copyright license out actually makes it less open source.

It's not required, but it's generally safer to put a notice saying who owns the copyright and what license the file is released under at the top of each file. Some licenses like MIT, the BSD licenses, Zlib, etc are short enough that you can include the full license text in the notice, and others like GPL provide sample copyright header text to include. Here's an example of this from a random file in the SDL source code: https://github.com/libsdl-org/SDL/blob/main/src/video/SDL_bl...

Obviously Microsoft is still committing copyright infringement and in the wrong here. However, if the author had copyright notices in each file and then Microsoft stripped them out or changed the copyright information, it would make it harder for them to brush it off with "oops, we forgot to commit the correct LICENSE file" like I'm sure they'll do here.

I would say: absolutely no (ianal). But I've had stand up arguments with colleagues in the recent past that I was unable to win. They wouldn't even ask the legal team for an opinion. But it's nice to see some evidence here that I was correct.