Originally posted by THUDandBLUNDERMost problem publications don't want problems with illegal positions, even for forced mate problems, selfmates, series, etc. The only exception is 'fairy' problems, which involve different pieces and/or strange rules anyway, so legality in terms of game history isn't as important.
IMO, it matters for games.
Less so for problems unless they are retros.
In fact, I think I will post a few of my favourites.