> many great projects have their last commit a year ago for example but everyone knows it's maintained
How does everyone know?
I think one year is not out of the actively maintained zone for a mature project, but that >18-month zone is problematic. Also the number and type of issues. I was just yesterday looking at such a Schrodinger's project and wishing I had a way to determine if it was still maintained.
Edit: People ALWAYS fix things when something outdated breaks the whole project.
I personally added some badges to all of my projects that inform the user if I abandoned the project and besides that, if a new feature from let's say Java is lunched and start breaking a project then why don't people raise an issue? It's not a PR and can be done in a few seconds. I'm sure someone will notice especially if a project is maintained by a handful of people.