Ellicott's Commentary for English Readers
(6) For for this cause was the gospel preached also to them that are dead.--This version is misleading, and seems indeed to be one of those rare cases where the original has been expanded by the translators for doctrinal ends. The Greek is simply, For for this end was the gospel preached to the dead also, or, still more literally, to dead men also. No one with an un-preoccupied mind could doubt, taking this clause by itself, that the persons to whom this preaching was made were dead at the time of being preached to. If this is the case, then, pretty obviously, St. Peter is carrying us back to his teaching of
1Peter 3:19, and is explaining further the purpose of Christ's descent into hell.