The bug

When the demo is over, you'll get a short text message:

You have passed your fifth day, use F2 to save a screenshot of your creation

The F2 keybind was hardcoded into the string previously. Now, with 18w20c, instead of the keybind, there are two curly braces: {}. Those are even less helpful than the hardcoded F2.

I would have expected there to be a %s escape sequence like with other strings where a keybind is inserted.

How to reproduce

1. Get a Minecraft account without buying the game
2. Log into the Minecraft launcher with that account
3. Start the demo world
4. Wait 100 minutes (or set the passed time to something like 119,980)
5. Look into the chat