Arxiving accepted manuscript after publication in Elsevier/IEEE/etc
I do not have a definitive answer, but I have considered the matter and came to the conclusion that you can only update an already arXived preprint, not arXive it fresh after publication. Note that as far as Elsevier is concerned, this is a recent change, which happened when they revised their whole copyright agreement. Thus decade long habits may not apply anymore. Or maybe they do, because if there is one clear thing, it is that Elsevier's policy has been significantly obfuscated.

However, there are some countries (France shall join them soon) where by law, one can post their postprints on a repository after an embargo whatever the copyright agreement says. Many publishers also allow this (but embargoes are several years long for some Elsevier journals, if I remember well).