if I were to ask for "what's the best tutorial or website to learn Drupal AJAX implementations" -- that's not allowed, right?
Right! "Best" is usually subjective, certainly with regard to what the best resource to learn something is (it will vary from person to person), which goes against our mission to produce a repository of objective knowledge. Asking for links has also been proven to produce generally low quality content across the Stack Exchange network, I'm not aware of any site that allows it.
This site is more for very specific, Drupal-only questions that don't involve PHP, Apache, REST, database issues, etc.?
If the subject matter of the question relates to Drupal, it's probably on-topic. If the subject matter is about something that you happen to be doing whilst also working on a Drupal site, it's probably off-topic. As a contrived example, not really an example of a "good" or even sensible question, but just for illustration:
I have Drupal installed in a sub-folder and I need to change the rewrite rule for the files folder. I tried xyz and it didn't work
Is a question about server config in the context of Drupal, it likely requires Drupal-specific knowledge to answer, so this is the right place for it.
I need to redirect all requests from the non-www to the www version of my site
Doesn't need any Drupal-specific knowledge to answer, it's the same thing for any website, so it can be dealt with elsewhere (i.e. Stack Overflow).
How do I change the colour of xyz element
Off-topic, CSS doesn't change with Drupal
I have this CSS rule that changes xyz element to red - how do I get it into an inline style in the head of the page?
Very much on-topic
There's a nuance to it that's hard to define 100%, so ironically there's an element of subjectivity in it, but only in the same way that literally everything in the world is rooted in someone or some people's opinions somewhere along the line. The guidelines in the help center do a good job of covering it for the most part.
Maybe someone with this type of requirement (recommendation) can ask this question in drupal.slack?
I can't speak to that channel specifically, but yes there are definitely other support avenues available than this site. We have a list of some of them here. If you find that Drupal.slack is a useful option, it would be great if you could provide an answer to that meta question with the details :)