No such restriction ever existed. AA and BA were allowed to codeshare, and they were allowed FF reciprocity, They opted not to because they didn't want to codeshare without revenue share. They did indeed codeshare outside of U.S.-London routes, including trains-Atlantics from Canada and Mexico City, where one could book tickets on an AA code and earn AA miles. AA was always afraid, rightfully so, that a full codeshare scheme would drive away premium passengers, especially because back then BA's product was vastly better than AA or just about any European airline. Most have since caught up.