If we're talking in terms of "widely practiced," there's a strong selection pressure favoring schools which don't impose activities on their students which are highly painful, frightening, or conducive to injury. In the absence of strong feedback mechanisms where if the training doesn't work, the students find out and leave, the most popular martial arts will be ones which don't demand much of their students.

A common adaptation seems to be to decouple the more painful and dangerous parts of practice from the less. I've studied a couple of arts where the harder forms of practice were taught in separate classes, at different times and not required for advancement in rank.

In both cases these were labeled "sparring class", but that's a little misleading; it seems common for them to incorporate more intense and painful conditioning techniques, or forms of partnered practice that aren't strictly sparring.