OED lists as:

    > Belonging to an earlier period, no longer in common use,
    > though still retained either by individuals, or
    > generally, for special purposes, poetical, liturgical

If "no longer in common use" or "for special purposes" doesn't mean incorrect in an entirely consensus-driven language, what does? It gives a motivating example of "obleege", for oblige, which I note my spell-checker has helpfully underlined in red.