I had not looked at the 'new' rules, but in the past, for one-class economy only flights (generally AeAgle); Y was the class to be used for A/DONEx.

If a flight had two cabins, one being economy, then L class was needed.

Looking at the rules now; this has indeed changed somewhat.

Even if L class is available, Y class can still be used on AA flights AA2500-AA5369 and AY/LA/XL/4M "Domestic" flights.

Other than those (and putting aside some JJ and JL flights) , L class is to be used when available at the time of booking.