flems.io (lightweight online html, js,css editor) does this also. i was very impressed, when i first encountered this. Everything is encoded in the url to share.
It's called a "fragment", and it's never sent to the server in HTTP requests. It was originally intended to have an HTML anchor as its value. Like when you click on a section or paragraph and you see the URL in the location bar gets "#<anchor>".
I looked it up: "#" is the location.hash and sets the anchor part of a URL. And is probably perfect for client side parameters, which is what I need. Such a discussion on HN is really often very helpful. There was a comment here that unfortunately got deleted again before I could reply that mentioned that.
https://flems.io/#0=N4IgzgpgNhDGAuEAmIBcIB0ALeBbKIANCAGYCWMY...
cryptpad does the same, i read yesterday. https://cryptpad.fr/