This is going to be an increasing problem, potentially even with large publishers unless suitable archiving arrangements are in place. Whether that is the case or not can be difficult to find out so any author who is worried should archive their work independently with an institutional repository, or PMC, or some similar venue.
As to the specific question, if the paper was OA then there seems no reason to not publish the work elsewhere. I would have thought adding a sentence somewhere such as the acknowledgements saying “An earlier version of this paper was published in XXX as [reference YYYYYYYYYY], but is sadly no longer available”, would satisfy any legal, ethical or moral qualms.
