A "chu" pronounciation seems to be used by most demographics in American society. To the point that most American films depict American and non-American characters using those contractions across space and time. From Star Trek's depiction of a fictional future, to Star Wars' possibly alternate universe, to Western films and shows. Is there a clear historical origin for this informal contraction in American society? It seems like its origin is early enough in United States' history that this pronounciation is used by most Americans.
I started pronouncing you-based contractions without a "chu" sound now that I've noticed it. It sounds too informal. I say that without any judgment. I'm a nineties kid and our generation has embraced casual and informal expressions of clothes and language among other things. However it now sticks out like a sore thumb to me. Follow up question: is there a generation or demographic that consciously decides not to use the aforementioned contractions in spoken form? I understand this might be a difficult question to answer so it's okay if it can't be answered.
I understand this might be a difficult question to answer so it's okay if it can't be answered.
Not difficult, totally answerable. Hi.
Is there a clear historical origin for this informal contraction in American society?
Yup, but not limited to American English. This is the result of what is called palatalisation.
It's the same process that results in "tunes" sounding like "choons" in some dialects. The cause of this is that when a sound like /t/ or /k/ precede a front high vowel or glide, so /i/ basically or /i/ sounding like a Y (/j/ in IPA), the resulting tongue positions to go from one sound to the next result in some overlap. This kind of overlap can be a kind of co-articulation, because you are essentially articulating two sounds at the same time.
It is also not even remotely limited to US English. You find it not only throughout English dialects, but also languages around the world. This change occurred in Mandarin Chinese as well. The name Nanking becoming Nanjing was not a change in name, but rather just the spelling reflecting an actual change in pronunciation resulting from the exact same process that makes "you" into "chu".
Sound changes also do not occur equally across all words. There's an old hypothesis that they did, but that's since been shown to not be the case. So in some places, certain words shift this way, in other places, other words do it, in yet other places maybe none of the words do it. This is just part of dialect differentiation over time. Distance + time are really all you need for dialects to develop and diverge, and there's not really a set course that they will take.
is there a generation or demographic that consciously decides not to use the aforementioned contractions in spoken form?
Generally no. There are exceptions, but most of this sort of shift in pronunciation from generation to generation is not conscious at all. You can think of it as the youngest generation fixating on features as important that older generations may have thought of as secondary. When you're speaking, you think you're producing a set of specific sounds, but you're really producing a whole heap of other stuff at the same time, be it small differences in pitch, in length of vowels etc. So you believe the important part is X, and are probably unaware that you're also doing Y, but the next batch of kids who are hearing you speak to your pears may be picking up on Y, and so that becomes the more prevalent feature. This is actually how tones developed in previously-nontonal languages as well.
But it's not a conscious decision when it's happening on a societal level. If a specific group of friends decides to do or not do something, that's different, but not really sustained anyway.
Hope that answers it for you. Why it happened that way in US English is just that "it did", and that's it.
If you have any more questions about this or want clarification don't hesitate to ask.