Last year I used a real simple web-app with my students so they would not have to drop their hard coded JSON objects inside the codebase of their JavaScript application.
Instead they could drop it into a textbox type of UI on this site and click the button and it would instantly provide an API endpoint and this way I could teach them how to create an API request to their data and it is super frustrating that I have since cleared my caching and browser history so I cannot find the name of the url.
It's a very humble simple site, its not jsonplaceholder, I am not asking for a JSON formatter. It has a grey background with a textbox where you just drop the JSON code and click the button and it gives you a working API endpoint that you can visit and see your data.