The Trials of Mana Steam demo is temporarily unavailable, because it seems like players discovered an exploit that unlocks the full game.
Steam made no mention of the exploit when they announced that they were taking down the demo, but it’s fairly obvious what happened looking at the timeline of events on Reddit, and it seems like a few days ago, someone shared a way to use the demo to access the entirety of Trials of Mana, and a short time later the demo disappeared from Steam. You don’t have to be a detective to work that out! (shout out to the Hot Fuzz fans out there).
It isn’t clear how long it’ll take for the demo to come back online, but it should go without saying that the bypass exploit won’t work anymore, and it seems like the progress made in the demo would have been saved, so you can pick up from where you left off once the demo returns.