This one is a more general physics post, but has some applications to parkour.
A funny quote that i cant seem to find the source of "speed never killed anyone, its coming to a sudden stop. thats what gets you."
thats actually a true statement, because so long as the speed is maintained or gently slowed the risk of injury is very low, but when speed is instantly accelerated to a stop, the impulse of the situation grows (the change in momentum over time increases.)
This applies to parkour because no matter what someone does, they need to come to a stop somehow, whether it break bones in the process ro not one needs to come to a stop (or else parkour would be more of falling through roofs than jumping over them.)
MAny moves in parkour and general gymnastics are made to dissapate momentum, such as the dive roll (which i made a post about already) or 'catting' a wall.
'catting' is gripping the wall with the edge of the fingers and hand so that while you are temporarily falling from the wall or surface, you can use your feet and arm strength to apply friction and stop before slipping. But at the speed at which the downward fall occurs leaves a very tine balance between apllying friction quick enough and too muych force on the ankles and hands (normal force of friction and the haanging force), or too much speed and slipping altogether.
Overall think about it, speed doesnt kill anyone, its sudden application of inerta and force against a surface that does them in.