Do we have to know the formal letter, or will they always give you a choice between formal and informal, if the formal even came up?
I think that you always have a choice😊
They don't always give you a choice
I would advise you to learn both. A formal letter hasn't come up since 2012 so it's definitely due. They are under no obligation to give you a choice when the formal letter appears. They can easily put just a formal letter on the paper with no choice.