true but in another book they are referenced as absent and not dead or disbanded and in another book they are just called the lost. So really there is no real way of telling what happened to them. Even the creators themselves don't call them dead only absent or second edition ultra marines codex they reference "it seems likely that Legions II and XI fought for Horus at least during the beginning of the Heresy" or something along those lines. or in Horus's vision where he morns the untapped potential that will never be.