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, during the COVID-19 pandemic. Shortly thereafter, the threshold of one billion public-domain citations from the Crossref database alone. Since 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.