That's just plain stupid I can't believe so many people think this would be a great idea.

1. There already are 3rd party programs that are able to do it so if you are too disabled to click print screen button you can download one of those.
2. This would straight up spam your PC, and as far as I'm aware games aren't able to delete your files without additional engine specifically designed to do so, meaning you would have to delete these screenshots manually, which most people would forget to do and collect gigabytes of pictures.

If you are so obsessed with this idea let me tell you there are way easier ways around it. For example a TEXTUAL log of all chats happening in game that would be sent to server at the end of the day and could be retrieved when a proof is needed, or ability to save replay which would include all chat that happened while fighting. Auto screenshotting is really most primitive and hardest to implement feature regarding proof when getting scammed.