I believe, the 20mA/LED figure has its roots decades ago, when LEDs were really not much bright. I still remember buying old, eastern block supplies of LEDs that were not happy unless you ran them at limit of datasheet values, especially green and yellow were poor. 10mA was barely usable, 20mA was OK-ish, 25mA was datsheet limit. I was teenager then (1990's), eastern block components were still plentiful; one LED cost me one day worth of pocket money, 4011 IC was three days worth and C520 (GDR equivalent of AD2020) was 18 days worth of pocket money.
For comparison, few years ago I bought huge pack of 1000 red 3mm LEDs from aliexpress (it was like 5USD or so), I tested them randomly, they are quite uniform, running happily and perfectly OK for indication purposes with 620Ohm resistor at output of 3,3V powered CMOS ICs.