I was elected as the treasurer of UCLU Anime Society 2016/17, and since I was a computer science student I was also responsible for IT in our society. Half way into the academic year we created a Discord chat for our society, and people begun requesting various features. One of said features was a bot that would somehow hide spoilers, which is how Discord Spoiler Bot was born.
Discord Spoiler Bot
Below you can see a quick demonstration of the bot in action. It deletes the original message and replaces it with a GIF that requires the user to hover over it to reveal content. If you haven't used Discord, to you this would sound like a very hacky way of hiding spoilers - well, it is! Sadly there aren't any other simple ways to achieve the same result.
You can find out more about about Discord Spoiler Bot on its GitHub page. By the way, it also works with long strings and by default trims content of the spoiler if takes up more than 6 lines, but this limit can be adjusted.
Finally, you can mark messages from other users as spoilers given you have sufficient permissions.