I'll here focus on the topic of questions that have to do with how the designers of a particular language make their decisions, since that was the motivation for the question here.
To be clear, I think the more experts we have here, the better.
But...
How big a problem is this really- given that a lot of this info is publicly available?
A whole hecking lot of design-decision-related discourse is made publicly available for a lot of well-known, well-established languages out there. Often, all it takes is the research effort to dig those up and clean them up for being answer-material.
With a lot of well-known, well-established languages, proposals for language changes are expected to present use-cases and rationales, those proposals are published and their histories are publicly tracked, and spaces made available for anyone to join in on the discussion. Take a look at
A lot of languages do conferences or design streams where they'll talk about higher-level design (where they later publish those as videos on YouTube, for example)
- Google "TC39" panel, and look at the list of videos in the search results. Those videos often touch on higher-level design principles, and how they apply to specific things the committee is working on.
- The CppCon videos are a goldmine- granted- it's still a mine- you have to dig. But one of the whole points of Stack Exchange is for some people who have done the digging to save others that digging work. Here's an example. And it doesn't stop at CppCon. Cpp.Chat is a great resource as well- hosted by a C++ Committee member, and often interviewing other Committee members on their current or past design work. There are more C++ podcasts that do these kinds of interviews with Committee members that provide this kind of info.
- Of course, the above point in interviews with language designers generalizes outside of C++. Here's a Zig example (you can find other interviews in that channel). Another channel I've seen examples of this on is Systems with JT (they have a playlist just for interviews).
Issue trackers and language blogs are another great resource. One example that comes to mind is generics in Go. They publicized why it wasn't a feature initially, a writeup about one of the proposals, and when they implemented generics, they explained why, and what costs it came at. Some language design groups even have a Discourse ("Discourse" as in the software that was created by the same people who created Stack Exchange).
- Even if the language design group doesn't have its own blog, sometimes individual members will, and will publish their thoughts through those channels.
There are other formats. For example, see https://github.com/tc39/how-we-work
What I'm trying to say is that often the problem isn't the information not being available- but just that it requires digging for.
Even if the information isn't available right now, that doesn't mean it will never be available in the future. Just let those questions sit. In fact- you can do more than that. You can go make that information available. Go talk to the maintainers wherever they can be reached. Ask them politely for their time. Invite them to answer the question here, or invite them to answer it on whatever public channel and in whatever format they are most comfortable with. Then, once the info is public, in whatever way is suitable for the Stack Exchange format and compatible with its CC-BY-SA license, answer the question.
Note- for anyone thinking that we should make these questions off-topic even after what I've stated above, take a look at some similar Stack Overflow meta discussion: Are questions about the motives of programming library developers on-topic?
