Synthetic caffeine has been commercially used in beverages and medicine for over 100 years. I believe that most caffeine consumed today is actually synthetic, but it's hard to find a good citation for its prevalence. Most of the web pages mentioning synthetic and natural caffeine are spam for "market research" reports or natural health woo.

You can see section "2.1. Production and use" for some relevant information about synthetic and natural caffeine production, as well as dated 20th century statistics about synthetic production:

"Coffee, Tea, Mate, Methylxanthines and Methylglyoxal"

https://www.ncbi.nlm.nih.gov/books/NBK507027/