What is this question actually asking and is it on topic?

Question

This question on the history of the use of 'plate' terminology in D&D has created some concern about what is being asked and if it's on topic.

I'm still unclear as to what OP actually is asking about and therefore can't even make a judgement on topicality.

A discussion to help clarify and then determine if it's in scope for the site should happen here rather than in comments or in a close-reopen war.

I'm not saying that it's either on topic or not, my goal here is to first understand what is being asked and then determine topicality.

Answer 1

There are different questions intermingled

There are at least four different questions asked throughout the post, having varying broadness and topicality.

1. The broad issue of plate armor names in 5e and how they came about.
2. The question whether the term half-plate is an invention of D&D, a modern invention (the term, not the thing) in general, or a real historical thing.
3. The question whether the term breastplate is used to mean the same thing as is does in the real world / if the D&D breastplate is the same as real world one
4. A comparison of plate armor names throughout the editions of D&D.

There are different issues with the different questions

1. The fact that you can read at least four questions from the post - of vastly different scope, no less - means that there is a problem of clarity.
2. The first question indicated above is potentially too broad to answer. Additionally, asking how these names came about is (most likely) asking about designer intent which would make it off-topic. The exception to both of these would be if we consider question four to be the actual question, i.e. "How did the 5e names come about from the previous editions."
3. Questions two to four look answerable individually, but putting them together into one question poses a problem of focus.

The actual issue and how to resolve it

The OP states in their question that the

question came to mind when [...] searching for historical examples of "half-plate" and found the term conspicuously missing from reliable sources

This indicates that they are actually interested in the term half-plate. They further state they have a similar concern with the term breastplate except in my opinion (feel free to disagree) the concern is not similar at all because the origin and historic use of the term is evident and the question is whether or not the D&D variant is historically accurate.

It seems then that (normally) the question should be narrowed down to one of the question here named two to four, probably number two. The other questions - if relevant to the querent - should be asked separately. In the meantime it seems to be reasonable to put the question on hold. However, there is already an answer answering questions two and three, therefore those should potentially be left together. This, however is a separate issue beyond the scope of the present inquiry and one that has been - as far as I'm aware - discussed before.

Answer 2

As written, it is off topic.

Asking the origin of these terms as they relate to D&D is unavoidably a question of designer's intent.

We can list all of the objective historical information about breast plates, but the moment we say, "And this is where Gygax/Crawford/Mearls/Perkins got their inspiration from", we are making an assertion that these people intended to represent those particular historical facts in the world of D&D. Asking if the game's designers were influenced by some historical truth is a question of designer's intent, which is off topic.

The history of breast plates in general is also off topic.

If we divorce the terminology from the game, and just ask, "what is the history of breast plates?", we are no longer asking an RPG question, we're asking a history question, which is better asked at history.se.

What is on topic?

The question as answered by Quadratic Wizard is on topic. Quadratic Wizard answers the question, "What is the development of these terms through the various editions of D&D?" This is on topic. But it is not what the question is asking. The question as written asks only about 5th Edition, and asks how the designers have related the 5th edition material to historical material - a question of designer intent.

The question needs to be reworded so as to ask about the development of the terminology through editions, otherwise it is off topic.