They self discharge and become inert, unless
Someone plugs them in. They won’t likely just catch fire, but there will be (is) a lot of dangerous to use junk out there.
There's an art collective around here that had a massive fire that started from a cell phone that had been held in a lost and found bucket for some years. Apparently it just caught fire one day.
How worried should I be?
But even if 1000 of these things burst into flames a day, that is still 0.04% chance of any given device burning up in a year.
Just please do NOT throw them in the garbage.