Are heatshrink tubings toxic for human health (e.g, using them in a "prolongedly-touched" place)?
Shouldn't be. Heatshrink tubing is just plastic. Certain plastics just shrink when heated just like the thin clear plastic wrap you see in that wraps tightly around boxes and other packaging. They don't really have to do anything weird to it to make it shrink.

The material used for most heatshrink tubing is polyolefin, in case you are curious. You might like to know there is also food-grade polyolefin heat shrink tubing as well. There is even medical grade polyolefin heat shrink tubing.