The stress on correct temperament is not new
From the FCI standard:
The Boxer should be fearless self-confident, calm and equable. Temperament
is of the utmost importance and requires careful attention. Devotion and loyalty towards his master and his entire household, his watchfulness and self-assured courage as a defender are famous. He is harmless with his family but distrustful of strangers. Happy and friendly in play, yet fearless in a serious situation. Easy to train on account of his willingness to obey, his pluck and courage, natural keenness and scent capability. Undemanding and clean, he is just as agreeable and appreciated in the family circle as he is as a guard, companion and working dog. His character is trustworthy, with no guile or cunning, even in old age.
The UK standard:
Character: Lively, strong, loyal to owner and family, but distrustful of strangers. Obedient, friendly at play, but with guarding instinct.
Temperament: Equable, biddable, fearless, self-assured.
The Australian standard:
The character of the Boxer is
of the greatest importance and demands the most careful attention. He is renowned from olden times for his great love and faithfulness to his master and household, his alertness, and fearless courage as a defender and protector. The Boxer is docile but distrustful of strangers. He is bright and friendly in play but brave and determined when roused. His intelligence and willing tractability, his modesty, and cleanliness make him a highly desirable family dog and cheerful companion. He is the soul of honesty and loyalty. He is never false or treacherous even in his old age.
The Canadian standard:
Character and Temperament: These are
of paramount importance in the Boxer. Instinctively a “hearing” guard dog, his bearing is alert, dignified and self-assured, even at rest. His behaviour should exhibit constrained animation. His temperament is fundamentally playful, yet patient and stoical with children. Deliberate and wary with strangers, he will exhibit curiosity, but most importantly, fearless courage and tenacity if threatened. However, he responds promptly to friendly overtures when honestly rendered. His intelligence, loyal affection, and tractability to discipline make him a highly desirable companion. Faults: Lack of dignity and alertness, shyness, cowardice, treachery and viciousness (belligerency toward other dogs should not be considered viciousness).
The New Zealand standard:
The character of the Boxer is
of the greatest importance and demands the most careful attention. He is renowned from olden times for his great love and faithfulness to his master and household, his alertness and fearless courage as a defender and protector. The Boxer is docile but distrustful of strangers. He is bright and friendly in play but brave and determined when roused. His intelligence and willing tractability, his modesty and cleanliness make him a highly desirable family dog and cheerful companion. He is the soul of honesty and loyalty. He is never false or treacherous even in his old age.
In many parts of the world (ie. the 50+ countries using the FCI standard), temperament is tested specifically and strenuously in a separate temperament test to the show ring, and a championship title cannot be awarded without passing it. There's an endurance test too, which helps to prove correct construction. In some places, dogs can't be bred without passing those tests.
But the rest of us rely pretty much solely on the conformation show ring. So the temperament displayed by dogs in the showring really is of the utmost importance and poor temperament really
should be severely penalised. So it is a good thing to see that written specifically into the US standard
As to whether or not temperament is evident as a puppy, then yes, to a large extent it is. A shy puppy is likely to develop into a shy adult. But this is largely a nature versus nurture question. And the answer to it is that the end result is a combination of inherent nature AND upbringing/experiences. This is why socialisation to a very wide range of experiences continuously from an early age is so important.
I don't think that an inherently shy puppy can be socialised into being an outgoing and fearless adult (though through socialisation there is no doubt that that dog will be far better than if not well socialised). But a puppy with correct temperament, if well socialised will continue to display correct temperament. A correct puppy that is *not* socialised will not necessarily end up shy, but is unlikely to display an outgoing and bold temperament either.
In short, you should choose a puppy that displays the correct temperament from the get-go. A shy pup can be improved through socialisation, and a correct puppy somewhat spoiled without socialisation - but a correct pup given adequate socialisation should remain correct. And it is the inherently correct pups that should be the ones considered for breeding the next generation, which is what assessment in the show ring is all about