Thank you for enhancing English on my side. But should not a story be as much understandabe, even for not native speakers, as possible? And he did not wote "mission critical" as agreed by you, but he wote "non-mission critical", where "not-critical for the mission" would be better, right? I wish you my level of awareness, while I wish (and working on) your level of English.
As "considered food" do not need "as", which other phrases are to be written and said witout the "as"? I consider this as quite important quesion.
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I am by no means an expert on the subject, so please take my answers with a pinch of salt (that is to say, take my answers with some amount of skepticism, as I may be mistaken). For reference, I speak American English. Also, if I am explaining things which you already know, I apologize. I am not trying to imply you are stupid or do not know something simple; I want to make sure I am being as thorough and complete as possible.
The phrase "mission critical" is used in military jargon as shorthand to mean "critical to the mission." If I said "The safety of the general is mission critical," what I mean is "The safety of the general is critical to the mission."
"Non-" is a prefix which means "not" or the negation of the word that follows it. For example, the word "Non-negotiable" means "Not negotiable".
So, the author is applying the prefix "Non-" to the phrase "Mission Critical" to create the phrase "Non-mission critical" (which means "not mission critical") This may not be strictly correct (you will not find the phrase "Non-mission critical" in an English dictionary). But, the use of this phrase works in the story because the story is written in first person, using the character's voice to narrate the story, so some intentional grammar mistakes can be made to convey the character's voice, as most people do not speak with perfect grammar. But, the writer needs to be careful that they are not making the text confusing or difficult to read.
In this story, the main character/narrator is a soldier, so the use of jargon like "mission critical" or "non-mission critical" works in the story because it makes the narrator sound like a soldier and it is still understandable as long as the reader is familiar with the jargon being used or is able to intuit the meaning. In this case, it sounds like you were able to intuit the phrase as meaning "not critical for the mission," so it seems to have worked.
Regarding the use of "as", I am afraid I do not have much information to give. In common usage of English (and, I think, most other languages), some words are removed or "dropped" to save time and space when writing or speaking. For example, I started this paragraph with "I am afraid I do not have much information to give" instead of "I am afraid that I do not have much information to give." Including "that" in the sentence does not change the meaning, add any information, or make the sentence easier to read, so I did not bother including it.
When a word can be "dropped" depends on the sentence, so I cannot give a complete list of examples or circumstances. But, in American English, "as" often is not included after "considered". For example: "Dogs are considered man's best friend," or "This sentence is considered grammatically correct English."
I apologize for the long reply and I hope this is helpful to you!
Have a look, he wrote "non-mission critical" and not wrote "no (space) mission critical" nor "not (space) mission critical" nor "no (space) mission-critical". There was no "mission-critical" written together, while there was "non-mission" confusingly written together. Am I clear? Read the sentence and make sure, that you really see what about I am writing.
I understand that "not" or "non" can be used, but I do not know about any usage of term "non-mission" or "not-mission". Maybe "non-mission equipment" can be said, but why for God sake?
Even if he would wrote more correctly "non mission critical, even more understandable would be "not mission critical" and the best woud be "no(t) mission-critical". Because "critical to a misiion" is "mission-critical" written together, while "mission ciritical" written not together (can) mean that the mission itself is critical. For eample, I made a TRRPG focused on relatioships, so it is relatioships-related (written together to be clear, athough ""relatonships related TTRPG" would be understandable too, but I am quite sure that it would not be so clear nor gramatically genial).
Even simple soldier would not be making mistakes (even not just gramatical) in such mission-critical terms as are "mission-critical" and "not mission-critical" or "mission-noncritical" etc. So I am not buying it as (inner) monologue nor as military jargon.
Anyway, for me, reading that story was like driving on a road with few bumps, therefore my rating for it was slightly worse. But I rated it still nicely, not?
I understand, that "that" can be removed, especially in simple sentences, including "if" type of sentences. But in my native language, we are conecting "consider" always with "as". So for me, who is still thinking mainly in my native language, the idea of dropping "as" after "consider" feels wrong if you catch my drift. If I understood you well, I can drop "as" after "consider" anytime when the "as" does not feel really neccessary?
I even literaly liked your reply and I am apologizing for this looong lament. Feel free to discuss with me Czech language (native for me) or - wait for it - SANSKRIT!:) (So English is precisely my TERTIARY language - heh.).
I've seen both "mission critical" and "mission-critical" used, and either is correct. It is in common usage in American English, so it is perfectly reasonable to use. Searching "non-mission-critical" on Google brings up results and definitions with both that usage and "non-mission critical", so I believe either can be used. "Not mission critical" would also be correct but sounds slightly more formal. As I said, "Non-mission critical" means the same thing but sounds more informal and soldier-like.
"No mission critical" would be grammatically incorrect, as "mission critical" is functioning as an adjective in the sentence, and "No" is used to negate nouns, not adjectives. For example: "There are no cats," vs "The cats are not present."