Since medicine has made ANY difference to survival only since 1920, and a great difference only since about 1940, that leaves more than 10,000 years of civilization in which crowding, new diseases, etc., have introduced greater natural selection effects, not less.
You seem to be forgetting that even two hundred years ago, the human population was much smaller. Better nutrition explains the population explosion better than anything. Even a hundred years ago, many people in London were malnourished. Interesting to read Charlie Chapalin’s autobiography. He was small because he grew up without much food. The Japanese today are much larger than their great grandparents.