Well the hard thing about astroturfing is that only the people running the platform have the hard data to prove it beyond any reasonable doubt.
But I don't think we need 99.99% confidence -- isn't even acknowledged that 30% of twitter is bots or something? I think it's safe to conclude there's astroturfing on any significant political issue.
Also as far as documented cases, there were documented cases of astroturfing around fracking [1], or pesticides [2]
1. https://journals.sagepub.com/doi/10.1177/2057047320969435 2. https://www.corywatson.com/blog/monsanto-downplay-roundup-ri...