Arrange a rack in the middle of the oven and heat to 250°F.

Place the bread in a single layer on a rimmed baking sheet and toast, stirring halfway through, for 20 minutes. Set aside to cool slightly.

Generously coat a 6-quart or larger slow cooker with cooking spray or butter. Arrange the toasted bread evenly in the slow cooker and set aside.

Place the eggs in a large bowl and whisk to combine. Add the half-and-half, pumpkin, sugar, pumpkin pie spice, vanilla, and salt and whisk to combine. Pour the custard over the bread. Use a wooden spoon to gently stir so all the cubes are evenly coated and soaked through. Cover and refrigerate overnight, if desired.

If necessary, remove the uncooked casserole from the refrigerator and let it sit at room temperature while you make the crumb topping.

Stir the flour, sugar, pecans, cinnamon, nutmeg, and salt together in a medium bowl. Drizzle with the butter and stir until big, slightly floury crumbs form.

Sprinkle the crumb topping evenly over the casserole. Place a large sheet of paper towels over the slow cooker, making sure it completely covers the top of the slow cooker but does not sag down onto the casserole. Cover and cook until set, 4 hours on the LOW setting or 2 hours on the HIGH setting.

Right before serving, dust with powdered sugar if desired. Serve warm with maple syrup if desired.