I cannot find a way to successfully force the column headings in a list to wrap instead of truncate... using JSON. The only post I found is below, and that didn't work.
{
"$schema": "https://developer.microsoft.com/json-schemas/sp/column-formatting.schema.json",
"elmType": "div",
"style":{
"white-space":"wrap"
},
"txtContent": "@currentField"
}