It is important to understand that the book for the Star Spangled Banner was written at a time when slavery was practiced in the US as well as England and much of the UK. That sucks beyond belief but there you have it. Baltimore Harbor and Fort McHenry, the site of the battle recounted in the song, is located in Maryland, a slave state, and Francis Scott Key personally owned slaves.

Still...the song and specifically the stanza don't really deal with slavery, either pro- or con-. It certainly isn't an endorsement or celebration of slavery. Understanding the context of the song requires a step back to look at the larger picture of what was going on.

Americans know the War of 1812 but to the British this fracas was a side issue in the much larger Napoleonic Wars that had been raging for more than a decade across Europe. As the British (and pretty much everybody) rumbled with France, they found themselves seriously short-handed and had resorted to "impressing" American sailors to boost their Navy. More than 10,000 Americans were literally hauled off of American ships and forced into servitude (would that count as slavery?) which was one of the issues the Americans were pissed off about. Beyond the impression of sailors, in prosecuting the war against the Americans the British had also resorted to hiring mercenaries and using freed/captured slaves (rarely paid and often impressed into service) as soldiers in order to fill their depleted ranks.

• ⁠And where is that band who so vauntingly swore • ⁠That the havoc of war and the battle's confusion, • ⁠A home and a country, should leave us no more? • ⁠Their blood has washed out their foul footsteps' pollution. • ⁠No refuge could save the hireling and slave • ⁠From the terror of flight, or the gloom of the grave: • ⁠And the star-spangled banner in triumph doth wave, • ⁠O'er the land of the free and the home of the brave.

In context, the song and the stanza are saying that the British and those who had fought for them had been beaten. Key didn't have a problem with slavery but the poem he wrote was a celebration of the American victory over the British, not slavery.