Does height really increase after 20 only through yoga and stretching exercise?

No, but. Height is how you carry yourself, proper posture, wearing the right clothes for your body habitus. Stand straight, hold your head square, don't slump, look confident, shoulders back, head a bit back (almost like trying to make a double chin): you'll look taller, tougher, send a message to that oncoming intruder that says 'don't mess with me'. Yes, stretch away. Perception is everything.