I had a friend who liked to sulk around in a trench coat. He bought a grocery store donut and promptly tossed the receipt.
He was soon stopped by grocery security for theft. After some hassle they tracked down his receipt and let him go, but yeah that’s what donut receipts are for.
At one time there were browser extensions that allowed you to comment on any web page and allowed other extension users to see your comments.
The comments were hosted through the extension and not on the pages themselves.
Something like that would be possible but I don’t know anyone offering it now. I presume no one wants to moderate that.