To echo Akuperma, the Chafetz Chaim’s Kuntres is about not shaving with a shaver. He has a footnote in the beginning about why it is that in these lands we were Noheg, and Mosser Nefesh, to have a full beard.
Nobody requires it as a rule. In fact, there usually aren’t any spoken rules in the first place. You can find many older people of certain Chassidusen that don’t have beards.
The idea of a full beard is from the Zohar Hakadosh. I once saw a Teshuva from the Ben Ish Chai to a business man who felt he had to shave for his business’ sake. The Ben Ish Chai allowed him to do so, and said that the small amount that he leaves over Lehalacha are enough to be Meramez to what the beard is Meramez.