Even in the very strict sense of "ethical" (pretty much a simpleton's "Ethics == Law"), I would say that Abandonware is abolutely ethical to pirate.
By its own definition it's software that is not being commercialized anymore, so nobody "loses" (if you use the current intellectual property legislation to defined winning/losing) any copyright income when somebody else copies it without paying them because there are no options for those people to get it by paying - even by the most fantastical definition of it, it's not a "lost sale".
Now, if the copyright owners resume commercialization of it, then it stops being abandonware hence stops being ethical to pirate it under this definition.
That said, for me anything that's outside the copyright length in the original legilsation (14 years) before Disney bought themselves extension after extension until the current "lifetime of the author + 70 years" (which adds up to around 150 years) is absolutelly ethical to pirate (or if you want to ponder on the Ethics of it: "Is it ethical to obbey a Law or a change of it which was bought?!").