I think the offensiveness stems from the fact that 'slanting' has the idea of 'not straight', 'not level. It's tied up with the old racist attitide that little people with slanting/slant eyes, yellow skin and a silly sing-song way of jabbering must be inferior to upstanding. decently white-skinned people with honest-to-goodness eyes who spoke God's language - English.
The facts (that people from East Asia do not have slanting eyes, that their skin is not yellow, that literature was being produced in Chinese 700 years before woad-wearing warriors in Southern England were trying to fight off Caesar) did not seem to bother the people who held these strange ideas.
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