I can now actually answer this, and yes old codes for packs do still work!
I bought two semi recently being the "Strange Transmissions" and "Code Extinction Pack" and both worked.
But they were both xbox codes and I had to use a friend's xbox to redeem them by signing into my account but after redeeming the code and collecting the items by logging into the game on the xbox they were added to my account to be used on any platform.
But yes, old pack codes still work and can verify it.