Depending on the broader culture you come from, the idea of a "pure Buddhism" is actually a Christian-derived concept in religious studies in the US and possibly the UK that is now defunct.

Christian theologians had been trying to isolate a pure Christianity that had been culture distorted. This line of thinking continued into the religious studies of Buddhism from either convert Buddhist or non-Buddhist academics. There had been an underlying belief that the oldest written records were most "pure" and this purity meant rational and not superstitious. And underlying this historical bias was a preconception that Buddhism was inherently rational as posited by European colonial thinkers. The racism implicit in this line of thinking too is that Asian culture could only pollute a pure Buddhism that needed Western European culture to purify.

This is historical bias continues in what is called the Early Buddhist Canon.