This is already available in reverse: you can mark a repository as unmaintained in GH to let people know you wont touch it anymore.

My thinking is that someone forgetting about a project, not caring about it, or for some reason being unable to update it (e.g. losing access to an account or even being physically unable due to injury or death) might not update the project to indicate this; I think a lot of the wariness of older projects that haven't been recently touched is because it's hard to tell the difference between "no changes need to be made so I'm leaving it untouched but monitoring" and "no one is even looking at this and if a change needs to be made, no one knows".

Someone will then come up with "Maintaina-bot-bot", so that we don't have to bother clicking that button.

Programmers are lazy creatures.

The laziness (which I have no issues admitting I also have!) is exactly why I think having to explicitly act each time to indicate ownership is important; otherwise, it would be impossible to tell the difference between software that's maintained but hasn't needed updates from something that no one is maintaining anymore, which is the issue we have now.

I think a mix of your suggestion + https://news.ycombinator.com/item?id=32994453 might be good, no?

You can update copyright comments once a year.

You should only do that if you have changed something. Otherwise you are falsely extending the period before the work enters the public domain.