There's more.

IF the states which had actually left the Union, the Emancipation Proclamation would be totally irrelevant to them. IF those states were still members of the Union (as the Union said) because secession wasn't possible, then the EP might well be unconstitutional because it took property without process of law (see the 5th Amendment). Frankly, I'm glad that Lincoln did it -- "Just as I would not be a slave, neither would I be a master."

Now consider this, the 13th Amendment:

Section 1. Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.

Section 2. Congress shall have power to enforce this article by appropriate legislation.

Note that slavery and involuntary servitude are abolished, effectively ratifying the EP. HOWEVER, note the exception "as a punishment for crime." Strictly speaking, I think you could become a slave if you were duly convicted. I really, really, really, wouldn't want to get embroiled in THAT, though!

So...those in the US who, say, are convicted of threatening the President COULD POSSIBLY PERHAPS sold into slavery?