Open citation data can improve the transparency and robustness of scientific portfolio analysis, improve science policy decision-making, stimulate downstream commercial activity, and increase the discoverability of scientific articles. Once sparsely populated, public-domain citation databases crossed a threshold of one billion citations in February 2021. Shortly thereafter, the threshold of one billion public domain citations from the Crossref database alone was crossed. As the relative advantage of withholding data in closed databases has diminished with the flood of public domain data, this likely constitutes an irreversible change in the citation data ecosystem. The successes of this movement can guide future open data efforts.