According to the new rules issued on Oct 1 last, on an AONEx ticket, if a flight doesn't offer First or Business class, I should be booked into Y economy class. BA refused this request, booking me, as in the past, in L class. (The booking was made after Oct 1.)

In the event of a flight cancellation and the necessity to be moved to another flight, won't Y take precedence over L?

And for BA tier points and Avios, is there a difference between Y & L on a flight that is under 500 miles (LAX-SAN rtn)?

From pandaperth on the rule change:

Rule 5 the rule on voluntary downgrades to lower cabins has been slightly improved

Old Wording was:
For flights where First or Business Class is not offered or available, passengers may travel in a lower Class, in the applicable booking code for that lower Class.

The new wording is:
For flights where First or Business Class is not offered or available, passengers may travel in a lower Class, in the applicable booking code for that lower Class.

Where the applicable booking class for the lower class is not available,
Passengers travelling on First Class Fares may book Y class
Passengers travelling on DONE* Business Class Fares may book B class

And BA is saying that L is available, so L is what you get

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.

There is a difference for TP's but not avios.
Y earns full economy TP's (20TP for <2,000 mile segment) while L earns discount economy TP's (10TP for <2,000 mile segment).

And I now realise it also affects OnBusiness pts: 200pts in Y, 100 in L.

150, I think.