[golem14]

I have a couple of USB cables from the 2000's like this, some of them original apple cables. But most are fine.

While some show brittleless, more plstics goes gooey and tarry - especially some ABS coating that makes the material more grippable,like computer mice or binoculars.

[joecool1029]

With the new information from OP we might have an answer. My phone cables were in box with phone and battery, enclosed space. I believe the battery electrolyte decomposing over the years outgassed and caused the TPE to degrade. mayyybe small amounts of hydrogen fluoride.