“The state of OA: a large-scale analysis of the prevalence and impact of Open Access articles” (Feb 2018, PeerJ)…
“the most common mechanism for OA is not Gold, Green, or Hybrid OA, but rather an under-discussed category we dub Bronze: articles made free-to-read on the publisher website, without an explicit Open license.”
Of Bronze, “few studies have highlighted its role” [in OA]. “We manually inspected a small sample of Bronze articles in order to understand this subcategory more; we found that while many Bronze articles were Delayed OA from toll-access publishers, nearly half were hosted on journals that published 100% of content as free-to-read but were not listed on the DOAJ and did not formally license content (using CC-BY or any other license).”
Bronze was found to be at a whopping 47% of OA, from a one-week sample of Unpaywall-DOIs in 2017.