Who cares about that? The South practiced slavery when it was part of the Union, so that's irrelevant.
What does the Union declaration of War say about the issue of slavery? Does it mention it as a cause?
The Union never declared war. It was the position of the United States government that the states had rebelled against the duly elected government of the US. Lincoln, with the support of congress and most of the north, used his constitutionally delegated powers to call out troops to defeat the rebellion and restore the constitution as the supreme law of the land. Just as Washington did to the whiskey rebellion.
I know in another thread you state you have only been studying this for a few years. As someone who has studied the civil war, and history in general, both professionally and for pleasure, may I suggest you print and read every rebelling states declarations of succession. While doing this underline every instance of the use of the word slave, slavery, anti-slavery, and any other variations. Also underline every time the word tariff or tariffs or any other variations are used. Then count the underlined slave and tariff words and I’m positive you will find a huge disparity between the word count. With slavery and its variations grossly outnumbering tariffs. Perhaps that will give you a hint on why the states secceded.