Worth it to add footing insulation if there's already a thermal break?
That is an expansion joint you are seeing. The material is 1/2" thick and something close to old style Celotex or beaverboard really soaked in asphalt or tar. It isn't much of a thermal break, but it is better than the masonry surfaces touching. Although it is available wider, it is usually only 4" tall. Either a bearing surface or gravel is below the 4" level. It is still in wide use in sidewalks and such, but nowadays ridged insulating foam is used. I have used it back in the day to set the top level of concrete, it was handy to float the concrete to.