Audrey Smith and co-authors, in a study published in MIT's Quantitative Science Studies late last year, with still more evidence that APCs exclude authors from the Global South:

We found that for the overwhelming majority of articles published in the Mirror-Parent ecosystem, the Author Geographic Diversity of articles requiring APCs was significantly lower than that of articles requiring no fee. This was true regardless of whether the OA articles were published in the established Parent journals or the Gold OA Mirrors. The overwhelming majority of these OA articles also had lead authors based in high-income countries. Despite being based in countries nominally eligible for APC waivers, authors from middle-income countries published proportionately few OA articles, but authors in low-income countries published almost entirely subscription-only articles in Parent journals. Taken together, these results strongly suggest that APCs are a barrier to OA publication by scientists from the low-income countries of the Global South.

The paper's research design is deliciously clever: Smith et al. probe Elsevier's own, deeply cynical use of so-called “mirror” journals—whereby a hybrid journal is cloned, complete with editorial board, in order to create a sham, Plan S-compliant gold OA title. (To its credit, Coalition S has slapped down the shameless ploy.) The paper's authors are, in effect, turning OA lemons into methodological lemonade, by using the mirror-parent journal pairing as a kind of natural experiment:

[…] the ability to compare OA articles published in Mirror and Parent journals means that we can control for many of the factors influencing these decisions. Most notably, the journals in a Mirror-Parent pair have identical editorial boards, editorial philosophy, and publication priorities.