The earliest mention of a record I was able to find comes from outside of China. In approximately 1820 an English monarch was gifted a coat of Pangolin scales by the East India Trading Company. I am wondering if the Pangolin trade is relatively new in Central and Southeast Asia or is there evidence of trade dating back further in Chinese history? An article listed below mentions how the demand for Pangolin scales has degraded the animal to almost 80% of the population it was in the 1960s, but before that records seem to be scarce. If the trade is recent, what sparked a demand in Pangolin scales in traditional medicine so recently in Chinese history?
https://www.sciencedirect.com/science/article/abs/pii/S0378111914009792?via%3Dihub
https://www.nytimes.com/2015/03/31/science/a-struggle-to-save-the-scaly-pangolin.html
https://royalsocietypublishing.org/doi/10.1098/rspb.2018.1084
The medicinal pangolin trade is quite old. We find prescriptions using pangolin scales in Bencao jing jizhu (本草經集注) of c. 499, via material surviving quoted in later works. This book is based on late Han Dynasty medical books, so there is a good chance that traditional Chinese medicine was using pangolin scales by the 35d century AD, if not earlier.
The original source of pangolins for Chinese medicine was southern China, supplemented by international trade presumably after there was already well-established demand. For example,
notes that recent (i.e., recent in 1938, when this was written) laws protecting pangolins in Indonesia will put pressure on Chinese pangolin populations, and improved pangolin protection laws in China might be necessary to protect Chinese pangolins.
The English-language literature has only noted the pangolin trade with any regularity quite recently, but the trade itself is probably much older. Today the pangolin scale trade for medicine is controlled in China, and an important contribution to it is farming of pangolins, but illegal trade continues. Being illegal, there aren't any good statistics beyond confiscation of pangolin products.
Growing wealth pushed demand for pangolin scales much higher. However, judging by prices, the demand for pangolin meat (itself a partly-medicinal demand, although bypassing the medical profession and prescription) was a bigger danger for Chinese pangolins, with the meat from a pangolin selling for up to US$1000, compared to about $200 for the scales (itself pushed up from less than 1/10 of that not long ago, due to increasing scarcity). How much the Chinese mid-2020 ban on using pangolin scales (and other pangolin parts) in traditional medicine will protect pangolins remains to be seen.
For a recent paper covering the pangolin trade and the use of pangolin scales in traditional Chinese medicine, see