Define "never". A 50% chance in a given winner? A 1% chance? A 0,01% chance? A 0,0000000001% chance?
According to
this map, the highest zone in the western continental US (if you'd call them "continental") is Catalina and San Clemente islands, just off the coast of LA, as well as a couple LA beaches. On the east, it's the Florida Keys -
11a Marathon to Key Largo, 11b Key Largo to Key West. Parts of Miami are 11a as well; After the 11a areas comes 10b: Coastal Los Angeles and San Diego, the area just north of Yuma, and the southmost 5-10% of Florida.
11b means an *average* of 7,2-10°C. 11a means an *average* winter low of 4,4 to 7,2°C. 10b means an *average* low of 1,7 to 4,4°C. Note that the median zone in Florida is 9b, which has an average low of -2,9 to -1,1°C. Most of Florida does freeze. Only the southern tip and keys don't "average" a freeze - but their zone gets so close to freezing on average, you can expect the occasional year to go below freezing. Depending on your definition of "never", you'd probably include the western keys (there's not been a recorded freeze in modern history), but probably not the upper keys. Small parts of coastal Puerto Rico and Oahu, by contrast, have zones as high as 13b. That's an average low of 18,3 to 21,1°C. I'd call that a place that "never" freezes!
I don't think even a major globe-affecting volcanic eruption would freeze that.
Note that zones don't tell the whole story on what you can grow. For example, here in the Reykjavík area we're in a "warmer" zone than Oklahoma City, Nashville and Richmond. Yet it's hard to grow here even plants that grow in places like Iowa. Our minimum temperature isn't that low, but our winter is very long and windy, and our peak summer temperatures very low, which greatly limits what can grow here. Part of the reason I'm thinking about the concept of doing geothermal root heating