Videos
What is the difference between an abbreviation and a contraction?
What are contractions (words)?
What is the difference between a contraction and a portmanteau?
Contractions definitely aren't rude to use in informal conversations. It's difficult to say why anyone would change your text on SE network that way, but it definitely isn't usual.
The only reason I can come up with is that if you're not a native speaker or your English isn't good enough, someone was trying to help save your question and dramatically edited your original text while subconsciously replacing all the contractions with no more reason than their own habit.
The editor (Nag) of your recent post on Stack Overflow lives in India. In my experience, Indians rarely use contractions, even in informal speech. For formal speech, the difference in use of contractions is even greater between the US and Britain and India. See Table 3.52.
IMHO, the choice of formality/informality, along with other matters of style and voice, should be left to the original poster. "Fixing" their voice to read like yours just ain't right.
I know it sounds silly.