I have lots with liquid in them. They rebound quicker than foam. In some the liquid is in lots of little balls within the squishy. I'm concerned that even if they pass security they might explode in flight or warp in some horrendous way. I thought about taking a stress ball but i have no idea what's inside them.
Off to google “squishy with liquid.” Like I said, all the ones I have bought on eBay are strictly foam and no liquid. You mention the orbeez beads and they have water inside. Some stress balls are made just of rubber, or some elastic co-polymer, and no liquids. There are those stretchy gummy rubber toys in shapes like frogs, and bugs. Not all squeeze toys have liquids, but I would think, if you stick with the most well known squeeze /stress toys (regular balls, for example) that you won’t have any problems.