Is 'distinctives' an obsolete word?
TL;DR of my post: It hasn't fallen out of use enough/doesn't fit the criteria for obsoleteness, and the plural noun form is still used today.
No.
But before we can proceed, we must look at what makes a word obsolete.
“We rarely take words out of our dictionaries,” says Mary O’Neill, managing editor of the largest single-volume English language dictionary, the unabridged Collins English Dictionary, which clocks in at 2,305 pages. “This is especially true of our larger dictionaries. If we find that a word has fallen out of general use, or is not used as much as it was before, we usually label such words as ‘obsolete,’ ‘archaic,’ or ‘old-fashioned’ rather than deleting them entirely.”
This paragraph, from Mary O'Neill of the CED, tells us that words that have fallen out of general use have the possibility of being marked obsolete.
But does distinctive(s) match this criterion?
Well, on the OED page, it's marked as a Band 6 word, though this is probably for the adjectival form, is:
Band 6 contains words which occur between 10 and 100 times per million words in typical modern English usage, including a wide range of descriptive vocabulary. It contains many nouns referring to specific objects, entities, processes, and ideas, running from dog, horse, ship, machine, mile, assessment, army, career, stress to gas, explosion, desert, parish, envelope, and headache...
So this implies that it's a word used as much as desert, to name one.
If we head over to NGrams, the word, including the noun form, appears to be on the up!
But if we head over to pretty much all dictionaries, even Google's, it doesn't appear to be a word.
However, as distinctives is the understandable plural of distinctive, lexicographers won't remove words that can be drawn from other words.
From Peter Gilliver, Senior Editor of the OED:
“Because a word like livery is still current, we don’t mark the extremely scarce derivative liveryless as obsolete because it is formed from elements which are still current and could be re-formed at any time.”
This applies to distinctives. (distinctive + -s plural)
Here is a book example from 2007 where it's used as a noun. There are more examples, should you care to look.
Since distinctive as a noun is still a used word today (See NGrams again); not marked as rare or otherwise, we can draw the conclusion: