But the concept and the way it is often portrayed in the Abandonware realm, no it's not (technically) legal.
Show me any Abandonware site which says it is legal. Sorry, but what you claim is simply not true! It has been very common to claim Abandonware is legal until early '99, but nowadays, it's just a few "old*****" sites who still do.
Edit: the word w-a-r-e-z is somehow censored out automatically....