Humberto Debat and Dominique Babini, in letter published in Nature:

We share the spirit of Plan S to achieve full open access to scholarly publications (see go.nature.com/2hszsaf), but we disagree with its implementation guidelines. The plan’s design ignores more than 20 years of widespread experience in open-access publishing in many developing nations, as well as Latin America’s widespread ethos of free-to-publish and free-to-read research.