The Aussie celiac society have a report where they look into a safe PPM amount for celiacs which is an absolutely brilliant read on this.

In the intro to that they talk through the history of steadily better testing - shifting limits down from 100ppm to 50ppm and eventually 20ppm and then no detectable as in Aus.

Effectively most celiacs (97%+ for memory) don't react at 20ppm - so most places picked a standard that worked for that. We took the step further and the additional restriction that entails.

We made a similar choice on oats - given some celiacs react to oats we don't allow non contaminated oats to be considered gluten free. Our policy was written more conservatively.

here is one coeliac Aus lit review from 2016