Preheat oven to 425°F (220°C). Line a muffin tin with silicone or parchment liners.
In a small bowl or measuring cup, combine plant milk and apple cider vinegar or lemon juice. Stir and let rest for 5 minutes to curdle into vegan buttermilk.
Lightly pulse the rolled oats for the topping (a few pulses only). Mix the pulsed oats, date sugar, nuts, cinnamon, and applesauce until crumbly. Set aside.
In a large bowl, whisk flour, baking powder, baking soda, cinnamon, nutmeg, and salt.
In another bowl, whisk applesauce, maple syrup, vanilla, and the prepared vegan buttermilk.
Pour wet ingredients into the dry and fold gently until just combined. Fold in diced apples.
Divide batter evenly into muffin cups and sprinkle with topping.
Bake for 5 minutes at 425°F (220°C), then reduce to 350°F (175°C) and bake 15–18 minutes more, until a toothpick comes out clean.
Cool for 5–10 minutes before serving.