found this info, I'm assuming it was a cotton blend at least.
https://leathermilk.com/leather-conditioner-rot-stitching
Cotton Threads
Cotton is an organic thread. The thread's quality is actually quite good on its own - cotton is fairly durable, and is very cheap. Next to nylon, it's among the most common materials used for threads. Unfortunately, cotton has some weaknesses. Cotton absorbs water and conditioner, takes longer to dry, has food value for bacteria, and can degrade outdoors. This fosters an ideal environment for rot potential, especially when the cotton stitching. This problem can be mitigated, somewhat, when the stitching is made from waxed cotton. This type of thread has natural resistance against water and oils, and will repel fluids well. Although this thread will likely last much longer than other cotton threads, it can still degrade under weather and sunlight. Cotton is particularly vulnerable outdoors, and as such, it is scarcely used to make leather products.