non-issue vs dead letter

non-issue and dead letter both are nouns.

Word	Noun	Adjective	Verb	Adverb
non-issue	Yes	No	No	No
dead letter	Yes	No	No	No

As nouns, non-issue and dead letter are synonyms defined as:

non-issue and dead letter: the state of something that has outlived its relevance

non-issue (noun)	dead letter (noun)
the state of something that has outlived its relevance	mail that can neither be delivered nor returned
	the state of something that has outlived its relevance