## 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 D&D breastplates are the same as real world ones
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, nonetheless, means that there is a problem of clarity. 
2) The first question indicated above potentially too broad to answer. Additionally, asking how these names came about is (most likely) asking about designer intent which would making 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.