The list of armor proficiencies in the druid class description (PHB, p. 65, or here in the basic rules) states:
Armor: Light armor, medium armor, shields (druids will not wear armor or use shields made of metal)
According the Sage Advice article from March 2016:
Druids don’t lack the ability to wear metal armor. They choose not to wear it. [...] If you want to depart from your class’s story, your DM has the final say on how far you can go and still be considered a member of the class. (emphasis mine)
Does this mean that an AL druid can choose to wear metal armor, because it is not stated that wearing metal armor is forbidden?