Spray a 6-quart slow cooker with cooking spray or line with parchment paper.
In a large bowl, toss sliced apples with 1/4 cup sugar substitute, lemon juice, 2 teaspoons cinnamon, nutmeg, and salt until evenly coated. Transfer to the slow cooker and spread in an even layer.
In a medium bowl, combine almond flour, chopped nuts, shredded coconut, 1/3 cup sugar substitute, and 1 teaspoon cinnamon.
Pour melted butter and vanilla extract over the dry topping mixture and stir until the mixture forms crumbles.
Sprinkle the crumb topping evenly over the apples, covering as much surface area as possible.
Cover and cook on LOW for 4 hours, or until apples are tender and the topping is golden brown around the edges.
Remove the lid during the last 30 minutes of cooking to allow excess moisture to evaporate and the topping to firm up.