I am posting this because of a particular instance (an answer which was recently deleted, most likely because it was considered spam), but I am hoping that the discussion here will be useful for other cases in the future.
This question, tagged reference-request and asking for "a rigorous statistics course or textbook," currently has +6 upvotes, -0 downvotes, and three answers: Two are scored +3,-0 and one is scored +0,-1 and is deleted (and so is not visible to those with "low" reputation). So it seems that the question is acceptable and answering it is acceptable.
All three answers recommend material that seem to be created by people other than those who posted the answers. That is, it is not obvious that the answerers are involved in self-promotion. The deleted answer is recommending material presented in a website and the other two answers are recommending a textbook. The deleted answer was posted by a user with 1 reputation and the other two answers were posted by users with higher reputation (24.6k, 437). The user with 1 reputation is a member of eight Stack Exchange communities, having a reputation greater than 1 for four of them.
In my opinion, the deleted answer is not spam and should not have been deleted. I do not see how it is considered spam but the other two answers were not.
I see nothing wrong with the deleted answer, and it is possible that I will post a similar answer in the future. I want to understand how "spam" is defined in Mathematics Educators Stack Exchange so that I will not post answers considered as spam in the future.
Perhaps the moderator who deleted the answer can explain their reasoning?
As requested in a comment, here is the text of the deleted answer:
As a student of mathematics taking a statistics course next semester, I have been plagued with this problem. I have found a solution in "Lectures on Probability Theory and Mathematical Statistics" by Marco Taboga, which is a collection of many lectures and exercises from the website https://www.statlect.com/.