"Slavery was abolished" is a statement, not necessarily a premise

Correct.

"Premise" describes the function of a statement within an argument.

The sentence:

Slavery was abolished.

is simply a statement/proposition when standing alone.

But if I use it as the starting point of an argument:

Slavery was abolished.
Therefore former slaves were legally free.

then it functions as a premise.

So the same sentence can be:

• a statement by itself;
• a premise when used to support a conclusion.