Oddly, if you read the amendment, congress didn’t explicitly outlaw slavery.
Speaking of Corwin's amendment, right?
First proposed by Jefferson Davis in December 1860, it's purpose was to discourage secession by providing more explicit protection for slavery.
It passed with Democrat support and was signed by Democrat President Buchanan.
Lincoln only transmitted it to the states as required by the Constitution, saying he did not object because Corwin only made explicit what the Constitution already implied.