SpudFiles

So, I have two Sodastream bottles that have gone past their expiration date. I do NOT use them to make the bubbly water, I use the glass bottle to make the water, then I pour it over into the plastic bottles and store in the fridge.

Do I still need to discard my plastic bottles even though I do not, and will never use them to make the water?

Even if you're not using the plastic bottles to carbonate the water and only using them to store it, it’s still a good idea to replace them if they've passed their expiration date.
Sodastream bottles are designed to withstand the pressure created during carbonation. Over time, the plastic can degrade, becoming less reliable and potentially leading to cracks or leaks. Even though you're not using them for carbonation, storing water in expired bottles might still pose a risk, particularly if the bottles are exposed to any kind of pressure or stress (such as in the fridge).
To be safe, it’s better to discard expired bottles and replace them with new ones. They are relatively inexpensive and ensuring your storage bottles are in good condition will help avoid any potential issues.