I think nearly all over airlines have Y for the Ecconomy Class.

By the way the "M" of Lufthansa is not a very friendly designation. It can be traced back to the time when there was no different seating between the people paying full fare and discounted travel.

At that time the slightly devaluating term "M" für Minderzahler (=people payinmg less than the published regular IATA fare) was introduced.

In the nowaday age the better designation would be F for (Savings-)Fox.

But that letter is already occupied, isn't it