Yes, it is normal for bottles of beer (or any liquid with a high water content) to turn slushy when left in the freezer for too long or at extremely low temperatures. This phenomenon occurs due to the process of freezing and the properties of water.
Water freezes at 0 degrees Celsius (32 degrees Fahrenheit). When you place a bottle of beer in the freezer, the liquid inside cools down rapidly. If the temperature in the freezer drops below the freezing point of water, the water molecules in the beer start forming ice crystals.
However, under typical conditions in a home freezer, the temperature is usually set to around -18 degrees Celsius (0 degrees Fahrenheit) or lower. At these temperatures, beer and other liquids can remain in a supercooled state—cooled below the freezing point without actually freezing. When you disturb or agitate the supercooled liquid, it can rapidly turn into a slushy state, and the entire contents of the bottle can freeze almost instantly.
So, if you leave a bottle of beer in the freezer for just a bit too long, it can turn slushy when you take it out and open it. This phenomenon is safe and does not affect the drink's taste or quality. However, it's essential to avoid leaving beer in the freezer for too long, as the expanding ice can potentially cause the bottle to burst or crack, making a mess in your freezer.