Which protocol requires the reduction of ozone-depleting substances?

An international treaty known as the Montreal Protocol was created to shield the ozone layer from substances that could damage it. It was first agreed upon in 1987, went into effect on January 1st, 1991, and has since undergone numerous revisions.

The main substances that should be minimized or phased out entirely are the use of hydrochlorofluorocarbons (HFCCs), hydrofluorocarbons (HFCs), and chlorofluorocarbons (CFCs); other substances like methyl bromide and halons are also included.

The timetable for a given chemical's phase-out varies depending on whether a country is classified as developed or developing. For instance, developed countries had until the end of 1993 to phase out halons, while developing countries had until 2010. The Montreal Protocol is generally regarded as successful because the hole in the ozone layer is closing.

See this linked Socratic question on CFCs and this one on the effects of a damaged ozone layer on the planet for more information and to view a timeline of events.

The protocol that requires the reduction of ozone-depleting substances is the Montreal Protocol.