Preheat oven to 350°F (175°C). Grease a 9x5-inch loaf pan thoroughly with butter or nonstick spray, or line with parchment paper leaving an overhang on long sides.
Wash zucchini and trim ends. Using a box grater, shred zucchini through large holes and measure 1 1/2 cups packed lightly. Do not squeeze out moisture.
In a large bowl, whisk together flour, sugar, baking soda, salt, and cinnamon for at least 30 seconds until evenly distributed and aerated.
In a medium bowl, whisk together oil, eggs, and vanilla until well combined and slightly frothy. Add shredded zucchini and stir gently until evenly distributed.
Pour wet mixture into dry ingredients and fold together gently with a spatula just until no dry flour is visible. If using add-ins, fold them in now with a few gentle stirs.
Pour batter into prepared pan, spreading evenly to corners. Tap pan on counter 2-3 times to release air bubbles.
Bake on center rack for 50-60 minutes, rotating pan halfway through. Bread is done when a toothpick inserted in center comes out with just moist crumbs.
Cool bread in pan on a wire rack for 10-15 minutes. Run a knife around edges to loosen, then remove from pan and cool completely on rack for at least 1 hour before slicing.