I would have to agree with what Fivestar4everMVP said. Where would the money come from to set up and maintain the day-to-day operations of such a demo platform? Nobody would be depositing, playing, and losing money, so there is no profit for the creators. But you would still have to pay for the game development and listing, bug fixes, server costs, and other things.

Most casino games are luck-based. There isn't much that you need to learn. Many casinos already have demo modes for slots. And if it's not available on the site you play at, you can just google the name of the game followed by demo, free mode, or something like that, and you will get a bunch of sites where you can play with fun money. You need to understand the rules of the games you play. You can do that be reading the games FAQ and help sections. Some time ago, I wanted to learn how to play Baccarat and everything is explained when you start the game and read the rules really.