Short answers, pulled from the story.
The Montreal Protocol was agreed on the 16th of September 1987 and entered into force on the 1st of January 1989. It has since been amended multiple times, with revisions agreed in London (1990), Copenhagen (1992), Vienna (1995), Montreal (1997), Beijing (1999), and Kigali (2016), among others.
As of October 2022, 198 parties have ratified the Montreal Protocol, comprising 197 states and the European Union. This makes it the first universally ratified treaty in United Nations history. The State of Palestine was the last party to ratify, bringing the total to 198.
British Antarctic Survey scientists Joe Farman, Brian Gardiner, and Jon Shanklin published results in 1985 showing abnormally low ozone concentrations above Halley Bay near the South Pole. Their findings, alongside NASA's satellite images, were central to the Montreal Protocol negotiations. Earlier foundational research was conducted by Frank Sherwood Rowland and Mario Molina at the University of California, Irvine, beginning in 1974.
A 2015 U.S. Environmental Protection Agency report estimated that the Protocol will prevent more than 280 million cases of skin cancer, 1.5 million skin cancer deaths, and 45 million cataracts in the United States alone.
The Kigali Amendment, agreed in October 2016 and entering into force on the 1st of January 2019, commits signatory parties to reduce hydrofluorocarbon (HFC) production and consumption by at least 85 percent from their 2011-13 annual average. HFCs replaced CFCs but are powerful greenhouse gases. Developed nations face earlier deadlines than developing nations such as India and some oil economies, which have until 2047.
The 2022 scientific assessment projects the ozone layer will return to 1980 levels by approximately 2040 for most of the world, 2045 for the Arctic, and 2066 over Antarctica. These dates were revised from the 2002 assessment, which had predicted recovery by the middle of the 21st century.