Just off the top of my head, some six man programs I competed against and coached for that don't exist anymore (I don't think) would be:
Brenham Christian Academy (school closed)
Harmony Science Academy - Bryan (school still exists but I don't think they have a high school anymore)
San Antonio New Life (I think they closed but I could be wrong on that, but I know they haven't played six man in forever)
Huston Academy in Stephenville (I think they just stopped playing)
The Fulton School in Heath (Not sure on the why for them)
Great Hearts Monte Vista in San Antonio (I think they started six man and then stopped shortly after, but you might want to check on that one)