Someone declared legally dead is presumed dead. Such a person may or may not be dead. So I say 'legally' in 'legally dead' is an alienans adjective. What is the test for an alienans adjective?
Let 'FG' be a phrase in which 'F' is an adjective and 'G' a noun. 'F' is alienans if and only if either an FG is not a G, or it does not follow from x's being an FG that x is a G. For example, your former wife is not your wife, a decoy duck is not a duck, artificial leather is not leather, and a relative truth is not a truth. Is an apparent heart attack a heart attack? It may or may not be. One cannot validly move from 'Jones had an apparent heart attack' to 'Jones had a heart attack.' So 'apparent' in 'apparent heart attack' is alienans.
'Legally dead' is like 'apparent heart attack' if we replace 'dead' with 'dead person.' If Smith is a legally dead person, it does not follow that he is a dead person. If a legally dead person should show up at your door, you don't dub him 'Lazarus.'
These linguistic niceties, besides being intrinsically interesting, are sometimes philosophically relevant.
More on this in the Adjectives category.