Convict Conditioning is one of the more popular calisthenics systems. It offers standardized workouts for every athlete.
This system was created by “Coach” Paul Wade. He claims to have spent more than a decade behind bars. Since jails are social bubbles inside society, low-tech training has survived in there and to survice in jail you need to be fit. Despite the whole con-slang, there is not much to it. Wade learned his trade from other convicts, as they spend a lot of time in cells. Developing a training program without equipment was mandatory.
If you follow this reasoning, it is obvious, why “ancient training methods” could survive. He elaborates further, but I think you get the idea.
I have no clue if he really is an ex-con and honestly, I do not care. The content calisthenics-wise is solid and that’s why I can recommend it.