I don’t know of any current HTV (Heat-Transfer Vinyl) that is actually made from vinyl; it’s all PU (polyurethane).
I just spoke with the customer success team over at Siser North America, the makers of Siser HTV. Their logo has the byline “Heat Transfer Vinyl”.
There is no vinyl in their “heat-transfer vinyl”. It can be cut with a laser.
the term seems to come from the fact that the material is roll-based (like sign vinyl), comes on a backing substrate (like sign vinyl) and is generally cut on a plotter (like sign vinyl).
They (and many others offering HTV) do sell sticker/sign vinyl that IS PVC based. Just make sure that what you are buying is PU-based HTV and not sign vinyl. If the seller can’t give you that information, find another seller.