This message appeared on one of my answers.
That message is a post notice. I'm the one who added it. There are several types of post notices. The most used in our community is probably the controversial post notice, but it's not the only one existing.
This is on a question which has many answers that do not include citations.
The other answers actually cite personal experiences or make a reasoning around what it's written in the question. Your answer is quite different. It makes claims about correlations and probabilities of certain events without citing any source for these claims. For instance,
The probability of defaulting on student loans is inversely related to the amount of money borrowed.
Where does this analysis come from?
I have never added citations to my answers in the past.
This is an unfortunate attitude in a community dedicated to the academic world which, if too widespread, would put us exactly at the same level of anyone spreading fake news on Facebook. Many users do their best to provide citations for the claims contained in their answers, exactly as they would do in their professional capacity, because this allows everyone to analyse the origin of the claims, verify if there are issues in the way in which the claims were derived etc.
Indeed, we can do better. It's true, there are still many unsubstantiated claims around this site, but from what I observed along these years, many users respond positively when prompted, integrating their answers. When this does not happen, a post notice can warn the readers of the issues of the post. As I said in this comment, other sites, like Skeptics SE, are stricter about missing references and an answer like yours would be probably deleted without a second thought. We don't have such a strict policy and we usually don't delete such kind of answers, but I think that users have their right to demand citations when needed.
Why am I seeing this message?
You're seeing it because several users complained about the lack of citations in your answer, which even received a flag for this. It doesn't happen that frequently.
I do not remember seeing it before.
Yes, it was used before, for example attached to this question (italics mine):
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