Arnold Zwicky has a good Language Log post on the word inimicable (significantly rarer than its synonym inimical, but attested since 1805) and the odd fact that the language mavens didn’t start bashing it until quite recently, the first basher apparently being Bryan Garner’s Dictionary of Modern American Usage (1998). On the issue of its place in the language, Zwicky points out that “there is a very close parallel to inimical/inimicable, namely unseasonal/unseasonable, and here both variants are standard.” But once Garner decided the word “should be extinct,” Robert Hartwell Fiske jumped on the bandwagon in his Dictionary of Disagreeable English: A Curmudgeon’s Compendium of Excruciatingly Correct Grammar (2004), calling it “a nonword.” I think Zwicky’s conclusion is indisputable:
Once a proscription — even a silly one, like Dryden’s Rule, banning stranded prepositions — is in the marketplace, it tends to persist. But where do the proscriptions come from? Here, there’s an enormous amount of randomness: somebody in the usage community happens to notice something that offends him (it’s almost always a man) in some way — often because he views it as colloquial or innovative or regional or used by the wrong sort of people, occasionally because that’s not the way you do things in Latin — and writes or teaches about it. We then end up with a collection of personal quirks and accidents of history, a big grab-bag of assorted stuff. Speaker-oriented hopefully gets excoriated, while speaker-oriented frankly and so on get a free pass. Sentence-initial linking however is judged to be poor style, while sentence-initial linking consequently and so on escape the red pencil. I could go on like this for quite some time.
It looks like inimicable got by uncensured until recently simply because no one was particularly offended by it. Not any more.