Poly-cotton is cheap, easy to get and reliable where dedicated basting thread is probably now heirloom material.
Cotton basting thread is easy to find. It is, as Greger noted, made from short cotton fibres (with a starching agent added).Even though it breaks under direct pressure, it's stable enough.
I have wondered about its degradation over time, but I have trousers which were made in the 40s and 50s, where the edges have been hand-serged with basting cotton, and it's all intact. Whether it would survive hand-washing, with the starching breaking down, I don't know, but I'm going to find out.
There's an old trick tailors used to use for getting fresh blood spots out of garments, caused by pricking your finger. They would chew on basting thread and lay it on the blood-spot. It actually works and the thread doesn't collapse either. I think it's pretty resilient stuff really, apart from breaking under tensile stress.