As a writer, editor or proofer, what is your thought on contractions? When is it appropriate to use or not use it?
I write fiction. I do use contractions in conversations, because it sounds more natural. I try to avoid it outside a direct quote or a conversation piece. I’ve had proofers shorten it all. To me it just looks weird to have contractions in your fiction.
Any article writers here and what’s you take on contractions on non-fictional writing for your clients? Yes, no? 
I think omitting contractions makes a piece sound odd and stilted - we use contractions in our billboards, our messages to one another, our notes…why should books be any different? It’s a personal choice, and neither is wrong exactly, but I’d automatically feel like I was removed from the author’s voice if I didn’t find contractions in the text I was reading.
I don’t do any work until the customer has signed a contraction.
hihi
I’m with @thatwordchick in that for general readability - oops ability to be read easily - a protracted lack of contraction can make for a rigid sense of flow. I can’t see why one wouldn’t use some constabulations from time to thyme lest one come across as rather stuffy, formal, and tautological.
Contractions are a natural part of the modern English language - both in speech and writing. They allow the flow of language to sound and feel more natural. They add personality to communication.
However, there are definitely times when to use contractions and when not to use them. For me contractions are fine and often essential in casual writing, and that can include blog pieces, communication with friends and colleagues, for example.
Contractions also have a place in some formal writing, such as speeches and fiction writing - particularly when characters speak.
But contractions have no place in other types of formal writing, such as contracts, legal letters, etc. I proofread a court submission yesterday for a client that included a couple of contractions - they went immediately. The contractions destroyed any credibility the document had.
We were told in our Business Communication class to not use contractions in business writing or when communicating with clients in writing. So, I don’t in most cases. Sometimes I don’t even use contractions when writing on the forum as it has become almost second nature for me not to use them.
When I write dialogue in fiction I do use contractions, of course, unless I am making the language archaic on purpose to match a setting. Outside of dialogue, I don’t use contractions in fiction writing.
In non-fiction writing, I would use contractions if the piece was meant to be informal and colloquial.
It depends. I’ve proofread a letter recently, and while it was a casual letter, nothing legal, or anything the like, the contractions simply looked out of place against the overall style. A text usually should look “as if made from one piece” in the end. That could mean getting rid of all contractions, contracting anything that can be contracted, or mix’n’match.
Typically, writers, editors, proofreaders should be able to get it right, especially with longer texts. If someone provides just a short text, checking other existing material (and the nature of the text/entity/…) should provide clues. (Let us not get into the tangential discussion about how customers should either provide enough clues themselves or alternatively cover the additional time needed for the checking/how sellers should include it in their offers, though).
However, there are also proofreaders who desperately want to, need to, correct something, to p̶r̶o̶o̶f̶ prove their worth, even if there might be nothing to correct, and while there are awesomely awesome proofreaders who really improve things by whole levels, there are also proofreaders who ruthlessly destroy what writers, translators, editors did on purpose, and ruin style and atmosphere.
If you’re happy with a proofreader and think they have a good grasp of not just their job but also your style and everything, chances are that they do the right thing; if you’re not so sure, maybe ask them and see what they say, and if you can agree, or not.
You could also ask someone, or several someones (ideally the kind of people who might, or do consume your writing), to read that same text, once with, once without contractions, without any additional info (such as which one is your original text, which one the proofread one, and without any kind of evaluative wording), and ask how they feel about it. Proofreaders might feel one way about it, while the vox populi might feel different.
And lastly, your own feelings, of course, aren’t the least important; even if your proofreader or your test reader(s) would give you reasons you can understand, but it still looks weird to you, as the author, you could also decide that it’s your style or even trademark, and keep doing it, it’s your text, after all, and the “accept(all)/dismiss” feature is implemented in Word for a reason.
That said, of course, you shouldn’t fight your proofreaders, editors, or other helpful people over actual facts, or over things they can explain well enough to convince you… unless the facts are facts but you still know that all your readers will know that your character, or even you as the narrator, not weak in grammar is but a Yoda reference you made, and your helpful person can’t tell ST from SW. Or if you don’t know but don’t care either 
In summary, my take depends.
I completely agree that the absence of contractions is critical the more formal the document. (Ctrl+f shows the main Fiverr ToS has only one! A ‘don’t’.)
I’mn’t a proper proofreader, and I tend to write more fiction than non-fiction. I find that using contractions as a normal ‘tone’ helps to provide more emphasis when it is used. (You know the speaker is getting serious if they’re dropping the contractions!) In dialogue especially, and when used consistently, it can provide a depth to characters and can show character growth (in both directions).
According to the Longman Grammar of Spoken and Written English, which was produced after extensive evaluation of a large corpus, contractions are very common in fiction writing.
The grammar also says contractions involving be and will occur four times more than those involving have and would.
Finally, it says they’re very rare in academic and formal writing. So this seems to confirm what @english_voice said.