The soft PTE sleeve was, and still is, a cushioning layer to prevent abrasion between old acrylic wall (a little rough back then but much smoother now). Even with lubricant, there was irritation factor involved. Back then, the clear soft PTE back then also used as a preventative measure just in case the acrylic cylinder cracked or implode. Yeah. Not fun to have fragmented cylinder chopped your penis apart under high pressure.
In the recent day, silicone sleeves are designed to act as compressive assist than to protect the penis. Acrylic materials are much strong today than 10 years ago.