When doing contract work, you don't normally get credit for a site on the about page. Sometimes you may get permission to add a footer link. I think this is great for those situations where you don't get either. My hope would be that most companies would not mind a simple text file in the root which isn't linked anywhere but is well-known.

My portfolio of sites becomes verifiable. Yes, I built these sites, and here are the humans.txt files to prove it.

I've been at plenty of companies and built many sites and can't really prove I had hand in putting building any of them.