The idea that expletives are not normal words is wrong. Common people have always spoken plainly. They would not have called their asses "bottoms" etc.
Given that auto-correct is a function of software then yes, word selection is part of Apple's business. There are two parts to the solution. The technical aspect is probably not at issue. The socio-political component is going to reflect mainstream corporate culture and probably not meet many corner cases. The significant choices aren't Apple's to make sense they will bow to the anathema dictates of social and political power: such as Winnie the Poo in certain Chinese contexts or Swastikas in German ones.
Like adjectives, nouns, verbs, adverbs, or any other of many word categories?
They are normal words understood by everybody, if not used by everybody, unlike your medical jargon.
But they're not normal words. They're expletives. If they have their own category, they're not "normal words."
iOS doesn't know a lot of the medical words I use for work, either. But I don't moan about it on social media.