Re-writing the phrase to "Hey, stupid..." was just to clarify the intent of the sentence, so changing "instruction" to "interjection" makes no sense. In the original phrase, it is still an instruction.

Yes, it would make sense; that’s how asyndeton — the rhetorical device of removing a conjunction and replacing it with a comma — works.

@Robusto — How so? We use asyndetic pairs of adjectives all the time.

Maybe, but they usually have a prosodic feel to them, consist of more than two instances in a row, and are not easy to interpret as something else. Which "Keep it simple, stupid" fails on all three counts.

Assuming that the Wiki origin described below is accurate despite being unsourced (big ask so it needs to be checked), this answer could be improved by including that origin and how it still doesn't matter. There's no way to make that formulation be understood by native English speakers without using and and, even then, it doesn't make much sense and isn't what almost all users of the phrase intend.

It may also be worth mentioning that the phrase is jocular tech slang: it's intended to be more familiar and less demeaning or patronizing (although of course that's an option too if someone's repeatedly overcomplicating something and leaving too many points of failure).

@ChrisA And it would give you the title of an Alice Cooper anti-drugs song as a bonus. :)

I can't accept this answer because the writer has incorrectly edited my question, changing its meaning completely, and is answering that question instead of my original one

@ChrisA "changing "instruction" to "interjection"," In your question, as it appears, you described the position of the comma as between "the instruction" (which is "Keep it simple") and the recipient (which is "stupid"). However, the only comma in the example is between "hey" and "stupid", and "hey" is an interjection.

@Greybeard look at the title of the question

@ChrisA So you have asked one question in the title and a completely different one in the body? And not only that, you have introduced the second question - the one in the body - with "i.e.", which indicates an explanation of the title text...

They're one and the same question. I'm not asking a question about "Hey, stupid! Keep it simple", I just rewrote it like that to give a clearer example of the vocative interpretation, in contrast to the asyndeton interpretation (which would be "keep it simple and stupid")

@ChrisA I just rewrote it like that to give a clearer example. I hope you now understand why that did not work.

@Greybeard I don't know why it's so difficult for you to follow.

There is no difficulty - I answered your question. Have another look at your title and the "i.e." in the body. It might help if this question is closed and you post a clearer one. Bear in mind that there is only one comma in your example.

There is only supposed to be one comma in the example. Where else would you put one? You haven't answered my question because your answer is still analysing "Hey, stupid! Keep it simple" (which is neither a common phrase nor in the title of my question) instead of "Keep it simple, stupid" (which is both)

@Greybeard OP has updated the question and it's perfectly clear now.

Excellent.........

@wjandrea I had already edited it by the time this exchange happened. That's why it baffled me that Greybeard couldn't understand it

@ChrisA I assumed they didn't notice the edit

@wjandrea I'd have thought people would double check before going off on one but there you go

@ChrisA re: "...the writer has incorrectly edited my question..." There is no record of the writer (of this answer, assuming that's what you meant) ever editing your question. Are you claiming that Greybeard somehow managed to edit your question without any trace of it appearing in the edit history?

@DavidZ look at the quotation of my question (indented) in the answer compared to my actual question

This has got busy since yesterday! I thought I commented on this question but can find no evidence I did. Just wondering whether the rewrite incidentally removes previous comments (none under the current question is older than 15 hours)...?

How has an answer consisting solely of an unnecessary rewrite, a ‘yes’, a ‘no’ and a false statement (“keep it simple and stupid” does make perfect sense) received 27 (at time of writing this comment) upvotes? This is not an answer; it utterly fails the how to answer test. It’s barely a comment.