Nunn-Lugar; see quick summary here: https://www.openphilanthropy.org/blog/ai-governance-grantmaking

Setting aside whether or not such risks were actually significant, perhaps planetary protection could be an interesting example of where bureaucracies spent time and money to mitigate unknown risks from e.g., extraterrestrial contamination.

The 2014 NIH moratorium on funding gain-of-function research (which was lifted in 2017)

The Asilomar Conference on Recombinant DNA, which Katja Grace has a report on: https://intelligence.org/2015/06/30/new-report-the-asilomar-conference-a-case-study-in-risk-mitigation/

Fixing the Ozone Layer should provide a whole host of important insights here.

The US decision to end their biological weapon program

The South African decision to end their nuclear weapons program and destroy their nuclear arsenal

The Nuclear Non-Proliferation Treaty (sort of; it's unclear how much of an impact it has had, but these posts [1, 2] provide some insight)

Some more examples of risks which were probably not extreme*, but which elicited strong policy responses:

• Y2K (though this might count as an extreme risk in the context of corporate governance)
• Nuclear power plant accidents (in particular Three Mile Island and Chernobyl)
• GMOs (both risks to human health and to the environment; see e.g. legislation in the EU, India, and Hawai'i)
• various food additives (e.g. Red No. 2)
• many, many novel drugs/pharmaceuticals (thalidomide, opioids, DES, Fen-phen, Seldane, Rezulin, Vioxx, Bextra, Baycol...)

*I'm not really sure how you're defining "extreme risk", but the examples you gave all have meaningfully life-changing implications for >10s of millions of people. These examples are lesser in severity and/or scope, but seem like they still caused strong policy responses due to overestimated risk (though this warrants being careful about ex ante vs. ex post risk) and/or unusually high concern about the topic.

Military/weapons technologies, in particular nuclear weapons, biological weapons, chemical weapons, and cyberattacks

Several infectious diseases, including COVID-19, Ebola, SARS, MERS, swine flu, HIV/AIDS, etc.

Gene-edited humans (see coverage of / responses to the twins modified by He Jiankui)

If you want to draw useful lessons for successful risk governance from this research, it also seems pretty important to collect negative examples of the same reference class, i.e. conditions of extreme risk where policies were proposed but not enacted/enforced, or not proposed at all. E.g. (in the spirit of your example of the DoD's UFO detection program), I don't know of policy governing the risk from SETI-style attempts to contact intelligent aliens.

Are you interested only in public policies related to extreme risk, or examples from corporate governance as well? Corporate risk governance likely happens in a way that's meaningfully different from public policy, and might be relevant for applying this research to e.g. AI labs.

I'm not exactly sure what you have in mind for the research, but I think it might be interesting to at least draw parallels or have pseudo-benchmarks with policy responses to non-existential low-probability risks, such as 9/11 (or terrorism more generally) and US mass shootings.