1049
Biology rule
(slrpnk.net)
Be sure to follow the rule before you head out.
Rule: You must post before you leave.
i just suggested it as a shorthand. the actual distinction is whether the word is generally used as a noun as well as adjective, and when it is, usually it's used as a plural noun.
it makes sense because plural nouns usually are a quick way to refer to a section of a population that share an aspect. but using an adjective as a singular noun has the connotation of reducing someone to that one aspect of them, which is the adjective. and so using an adjective as a noun with an -s pluralization implies there's also a singular form which is usually offensive.
language is fluid and it evolves, so nothing here is a hard rule and there will be exceptions, and things might change with time. this is mostly based on observation and convention.
I'm not convinced that there's even a soft rule; I think it's just a case of the one or the other way of doing it nebulously sticking, like how sometimes you form a noun with -ness and sometimes you do it with -hood. Which now I think about it is more or less what you're saying, but I don't think it's done consciously at any rate.
language conventions are rarely conscious. they just happen. every now and then there's a campaign for our against using certain words or phrases; sometimes they stick and sometimes they don't. but those are conscious i guess. mostly though it just happens organically.
like a perfectly normal word becomes vulgar in time if enough people just say it a certain way. it's not like people suddenly hold a meeting and decide this word is bad now. it just starts to feel like it after a while, so it eventually becomes so.