Physiotherapist and Osteopath Klaus Isele takes a look at improving the posture, starting with flat feet. Guest in this video is pro-climber Adam Ondra.
Many people have flat feet. Most doctors agree that flat feet does not do good for the biomechanics of the lower leg, it is better when your foot has some arch. In addition to a tip and an exercise provided in the video, I can add my personal (verified) tip: Pay attention to how you land your feet during running; try not to land your foot on the ground at once, instead start with toe and the outside/lateral part of the foot. In another words, my tip is similar to what Klaus tells Adam to do but applied to the case of running.
It is hard to say how important proper arch of the foot is for climbing. Adam says he feels the difference. From my observation and from common sense mechanics, you will stay better on small holds if your feet has some arch.
What I like is that professional climbers are becoming more serious about everything related to their mental and physical condition, including body posture. Put little things together and they can make a big difference. Pay attention to all parts of your body, know how a properly developed body should look like and try to grow your own body accordingly; at young age some things about your body can be fixed with enough effort. Hopefully climbing coaches will incorporate this knowledge into young athletes in training too.