The Trials of Mana Steam demo is temporarily unavailable, likely because 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.
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 need a degree in investigative journalism to assume there's a connection.
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 (not that anyone should use it regardless). And if you're worried the progress you made in the demo is gone, fear not - Steam has clarified that you'll pick up right where you left off once the situation is sorted.
Thankfully, you can still access the demo on PS4 and Switch, and you totally should. It's a meaty thing that gives you a couple hours of the story to enjoy, and everything carries over to the full game if you buy it. Granted, it's a very traditional, linear JRPG and might not be for everyone, but I'd say the demo is worth checking out, if only for a bit of free entertainment.