Thanks for the comments.
We could have extended discussions on where and how to represent morphology and etymology, but these comments are NOT the appropriate forum. Please feel free to raise an issue in the Github forum if you wish. 
https://www.edrdg.org/wiki/index.php/Editorial_policy#Issues_Forum

This is strictly morphology that the reader can apply to their use of the language, for example: knowing it is made from ん and what ん derives from allows them to correctly predict the existence of ませぬ - or - if someone learns of ぬ through ません and learns from ぬ's entry its function/meaning and how it is morphologically added to other words, then they would be equipped to locate and understand every single instance of ぬ or ん they find from thereon, as well as to use it themselves. Is this not a benefit for the reader?
This is in line with the editorial policy that lists "where there is a derivational relationship between words that it is useful to highlight, e.g. between かっけー and 格好いい, or between オケる and 空オケ." as a cross-reference that can enhance the value of the entry; this is fundamentally the same as the given examples - and with ぬ being a productive morpheme, morphology notes and references would be even more useful.
Now, I do agree that it is not all etymology and/or morphology that should be added: here there is a positive impact for the learner's understanding and practice. Contrast this with something such as adding "changed over time from まゐらす to まらする, まする, culminating in ます" to the ます entry. The latter would be a truly useless historical etymology that does not help readers to understand what something means, how it comes to mean it, nor how to use it.


I understand that the dictionary is not here to teach morphology, but it is here to teach Japanese and morphology just cannot not be useful in such a highly agglutinative language - in fact, it is not only useful, it is required for its use: it is the necessity of morphology that led you to add "after the -masu stem of a verb" to this very entry, it is morphology that dictates we have separate verb classes and it is morphology that distinguishes verbs from adjectives and tells you how to use each. 
If we are ignoring any mention of etymological and/or morphological information and concern ourselves only with self-contained semantic/functional minimalism, we might as well remove: the left half of the note in this entry along with all 3 cross-references, the cross-reference to 格好いい in かっけー, as well as any other similarly substantiated note or reference in any and all entries. We could even remove the tags for 動詞 and 形容詞 and neatly bundle them under a category for 述語 since that is the all-encompassing superset. Nonetheless, I think you will agree that would make the dictionary less useful as it would lose much of its power to teach usage - which directly affects ability to teach identification and thus, comprehension - and completely lose any information about derivative connections, all because of morphology and etymology.

Do you believe that usage, identification, and relations are not something that a cross-linguistic dictionary should seek to teach? Is it not what the reader seeks to learn? If you do think so, then what does it seek to teach? Raw semantic correspondence? If you give yourself the bother to think about these questions, consider: Every time someone conjugates something, they are engaging with morphology. Every time you add an [adj] tag to something, you are teaching them about it.

Since we're not attempting to explain the etymology.

How would the cross-reference to ん be redundant? As it is, there is no way for one to know that the ん in ません is the negative ぬ・ん from simply looking at this entry.

I agree with Robin on the etymology, etc. The xref to ん is quite redundant.

@@ -10 +9,0 @@
-<xref type="see" seq="2139720">ん・3</xref>