1) The drink has gone flat.
(If it's a flat drink, it never had bubbles)
1) The crisp has gone soggy.- This is OK to me.
1) The cracker was smashed.- I would use crushed here.
1) The toast bread has turned soft.- I would say The toast has gone soft. (I would not use toast bread- you can say toasted bread, but toast is enough and I see no reason to use more here)
Student or Learner