We preferred frozen peaches over fresh because of their year-round availability and relatively consistent levels of moisture and sweetness. Shrub syrup can be stored in an airtight container for up to 1 month. Shake well before using.
Bring peaches, sugar, and water to boil in large saucepan over high heat. Reduce heat to medium-low, cover, and simmer, stirring occasionally, until peaches are soft and beginning to break down, about 10 minutes.
Remove saucepan from heat and use potato masher to crush peaches until uniform in texture. Strain mixture through fine-mesh strainer into medium bowl, pressing on solids to extract as much liquid as possible; discard solids. Whisk in vinegar.
