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.

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.