The changing map below shows the states with slavery and those without from 1789 evolving to 1861!
The reason for the Oklahoma Panhandle is that it is north of 36 degrees 30 minutes North, the northern boundary of slavery according to the Missouri Compromise (except for Missouri itself). It had been part of Texas when Texas was annexed but in 1850 Texas was reduced to its present borders, none of which was north of 36 degrees 30 minutes because of the compromise of 1820. Ironically a few years later that rule was thrown out by the Kansas-Nebraska Act. Looking at the map it doesn’t make sense that what is now the Oklahoma Panhandle wasn’t just part of the Texas Panhandle.