Ellision? You mean "elision", don't you?
Yes, it is an elision, and all dialects, including standard British and American English, have them (think of all our apostrophes).
But it's too simplistic to say that it's "not well-regarded". It is very well-regarded by some people; indeed, if you mixed with certain people in a certain setting and said "shut up" in perfect BBC English, you would probably be mocked.
This doesn't necessarily mean that people should use non-standard dialect forms all the time. Of course, if you are applying for any kind of job that involves more than manual labour, you'd be advised to use forms closer to the agreed standard, which in Britain is an artificial dialect based loosely on the dialects around Oxford and parts of London; and in America is similar to a dialect spoken in parts of Iowa, Nebraska and Illinois.
These dialects are considered to be "superior" not because they are better than any other dialect but because they became the de facto national standard by historical accident: "General American" because radio broadcasters preferred to hire speakers with that accent; British RP because it was spoken in and around the important institutions of government and seats of learning.
As it is, speakers should use the standard dialect, or as close to it as they can manage, to communicate with people from other areas; if you are, for example, a secretary, speaking in Scots English to a client from Basingstoke would not be good for your career.
All that aside, you would be well advised not to utter the phrase "shut up" in a job interview, regardless of what dialect you do it in.
Is there a word shurub in Engish ? 
- however, it is still an ellision and not usually well-regarded. 
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