"…actually overplasticized PVC,"

Despite the point in my comment about power cable extensions coverings becoming hard through the lack of plasticizers, I confirm your point about overplasticized PVC in instrument cables.

I have a lot of instrument leads that I thought were silicone only to find that after a year or so they've gone hard. So much so that new unpacked ones can no longer be easily straightened (the kinks remain unless one heats the cable up and then straightens them).

Despite availability issues,the problem is trying to determine what's real silicone and what's not.