The top 10 paywalled articles are all from the 20th century. The Open Access movement is great but it doesn't do anything to free up papers from the past. A la

The top 10 paywalled articles are all from the 20th century. The Open Access movement is great but it doesn't do anything to free up papers from the past. A large part of the problem is the ridiculous duration of copyright. "Adsorption of Gases in Multimolecular Layers" is from 1938 and still paywalled. In practice, almost all papers this popular will be available on random .edu sites and Google Scholar will find those technically-forbidden copies for you. But it is a significant problem if you don't have an institutional affiliation and you want to read articles that aren't among the top 5% cited. (Or at least it was a problem for me before sci-hub; I retained academic contacts who could email me any papers I wanted, but I had to cross a pretty high interest threshold before I'd bug someone to request that favor.)