I was looking at this question, "Are there any keyboards that can let you do pitch-bend by directly manipulating the key," which has attracted several close votes as a "recommendation request." I chose not to vote it as such, though; to me it's an organological question. It happens to be geared toward modern, commercially-sold products, but the OP never once implied that they were seeking to purchase, just looking for knowledge. It seems to me little different in spirit to, say, "Is there a bowed string instrument with strings in paired courses." Meanwhile, the prototypical bad "recommendation request" question is "Which practice amp should I buy." There are some objections that "bowed strings with paired courses" has in common with that one, and some that it doesn't; clarifying which objections are central to our ban could help inform close votes.
This seems to be one of the most relevant meta pages on "recommendations." It mentions that "Questions like this tend to turn into specific equipment/product recommendations, yield link/product only answers that may not be valid long after the post, attract spam and people advertising their own product, and tend to focus more on products." Paraphrasing those points or adding to them, some possible reasons for this objection could be:
- Prone to answers that are not substantial, or are even "list answers." To the question "are there instruments with sympathetic strings," the answer could be "Sure; sitar, sarod, viola d'amore, hardanger fiddle...". This could be a problem with even organological questions, the "does [this instrument] exist" questions, although I have created a straw-man question that's excessively broad and poorly researched. My earlier one, "do any bowed instruments have paired courses," could have much more substantial answers, unless the answer is actually "nope, sorry." (Man, now I'm curious... At any rate, such questions can be made to demand more substantial answers by also asking "and where/when/how was it created/used/etc."
- Attract opinion-based answers. "What is the best practice amp" can have multiple equally-valid opinion-based answers. This is not a problem with "does X exist" questions.
- Concern about commercialism. "What's the best amp" questions might attract actual spam, or raise concerns about undue favoritism on our part. These would not be concerns for organological questions that deal with "types" of instruments ("The answer is a clavichord"), though they could if the answer is a commercial brand or model ("The answer is the Seaboard Rise 2").
- Focus on means over ends—talking about musical tools when we should talk about problems to solve. The "is there an app that teaches me ear training" question restricts the conversation to apps, when perhaps the best solution is a different type of resource or even holistic approach. This is irrelevant to "tell me about X instrument" questions, unless they conflate practical concerns into them ("Is there an instrument designed to imitate bird calls? Because I want bird-call sound effects in my piece.").
So which of these concerns underlie the opposition to "recommendation" questions, and how strongly does each contribute to that ban? The answer would help inform CVing, among cases edge-ier than "what's the best amp."