Preheat oven to 350 degrees F. Lightly grease two 9x5-inch loaf pans.
Stir sourdough discard in a bowl. Let sit until bubbles start to form, about 5 minutes.
Meanwhile, whisk flour, cinnamon, salt, baking powder, baking soda, ginger, and cloves together in a medium bowl until well combined.
Combine pumpkin puree, mashed banana, brown sugar, oil, molasses, and vanilla extract in a separate large bowl; whisk vigorously until brown sugar is completely incorporated with no chunks remaining. Stir in sourdough discard until evenly distributed. Add flour mixture gradually, at first whisking to combine and then switching to a heavy spoon and stirring until just incorporated; batter will be thick. Stir in raisins and walnuts and spoon into the prepared loaf pans.
Bake in the preheated oven until a toothpick inserted in the center of each loaf comes out clean, 60 to 70 minutes. Remove from the oven and let cool in the pans for 5 minutes. Transfer loaves to a wire rack and cool completely before slicing, about 30 minutes.