In Rian Johnson’s mind, Captain Phasma could be dead and Finn killing her symbolizes that his character has finally committed to the Resistance and the fight against the First Order.
That’s the tricky thing with there being no overarching plan for this new Star Wars trilogy ahead of time.
My interpretation of The Last Jedi was that Finn likely killed Captain Phasma when he knocked her into a fiery chasm while the Supremacy fell to pieces.
But much like with Rey’s parents, J.J. Abrams could decide while working on the script for Episode IX, that he’d like to use Phasma again and devise a way the character survived.