Decided to make a chart, this one is through the end of 2022, showing what the USDA zone calculation would be from 1977 to present (data starts in 1948, but the 30-year average can’t start until 30 years later):
Can see that the 30-year average has been going up now for the last 40+ years, but didn’t cross from zone 8b to zone 9a until around 2013.