Applying resource-recommendation/link-only policies

Question

In principle, questions tagged resource-recommendation that do little more than provide a link should be deleted like any other (almost) link-only answer, but is it really so? If we look at most answers to, e.g., the super-question Book recommendations, we see that most of them have a simple one-paragraph description and a link, which indeed seems fine for the purpose.

As a concrete case, recently a resource-recommendation answer was deleted on the basis of being link-only (this, for those who can see it):

[user207599] Here is a Kerr metric derivation method by using ellipsoid coordinate transformation. https://academicjournals.org/journal/IJPS/article-full-text-pdf/AEE776964987

While similarly long answers to the same question have been left alone, namely:

[Robin Ekman] The original reference is Kerr, R.P. Gravitational field of a spinning mass as an example of algebraically special metrics. Phys. Rev. Lett. 11, 237.

[Mohamed IBrahim] you can also read this paper , it is very useful A Possible Intuitive Derivation of the Kerr Metric in Orthogonal Form Based On Ellipsoidal Metric Ansatz.

[Paul Masson] I'm surprised no one mentioned Chapter 6 of Chandrasekhar's Mathematical Theory of Black Holes. Then again, I personally found his derivation so opaque that I wrote my own de novo derivation in Boyer-Lindquist coordinates using the Cartan formalism.

[fazlay ahmed] i think if we follow the Ray D' Inverno book intitled Introducing Einstein Relativity then we get easily this metric. on chapter 19 it is explanied in a very simple method

So should all these four answer be also deleted as being link-only? One of them has 4 upvotes, two have 1.

A closely-related point: the policy for resource recommendations states that:

If you come across an old recommendation post, move all of the answers that are against policy into a community wiki list answer, and give it the following banner [...]

This answer is from Aug 5 '13, so "old" means earlier than that?

In summary, my questions are:

1. Is the link-only policy loosened for resource-recommendation questions?
2. We should only agglutinate link-only resource-recommendation answers if older than Aug 2013?

Answer 1

1. The paragraph you refer to applies to answers that were posted before the resource-recommendations policy was set in mid-2013. After the policy was agreed on, there was an extended campaign to find and fix as many old [books] questions as we could, and bring them into line with the new standards. (You can find many of them using this query as a starting point.) Basically, the agreement at the time was that the close-every-book-question-on-sight policy would be relaxed, in the understanding that we managed to put together enough of a community curation of new questions as they came in and of the existing corpus.

2. That campaign to fix the old answers was quite successful, but we still missed some (as you have found). If you find them, process them as indicated in the policy. Most of it you can do yourself, though some answer deletions do require raising a custom moderator flag to say "hey, I've aggregated all the old answers into a single catchall one, you can delete this one now." I've gone ahead and done all the user-level processing of the thread you mentioned. Hopefully the mod-level answer deletions will come in shortly.

3. As for the answer you mention, it was posted a full four years after the question started sporting a banner that states

   Answers containing only a reference to a book or paper will be removed!,

   bold font and exclamation sign and all. Feeling sorry for the deletion of a low-quality answer posted in those conditions is pretty moot, I think.

4. It's worth pointing out that there is another deleted answer on that thread; the review link is here. There's some familiar faces there.

5. For full clarity, when the policy says

   If you come across an old recommendation post, move all of the answers that are against policy into a community wiki list answer

   it's referring to answers posted prior to the policy itself. It's now been several years since that, so depending on conditions it might be appropriate to extend some amount of leeway to answers posted, say, on 2015. But frankly, if an answer was posted after the banner, then it has it coming, and it should be deleted.