To make the meatloaf: Stir breadcrumbs and milk together in a medium bowl and let sit for 5 minutes. Add beef, onion, egg, thyme, salt, and pepper; stir to combine evenly without overmixing.

Preheat an air fryer to 350 degrees F (175 degrees C). Meanwhile, form beef mixture into 2 (3- x 4 1/2-inch) rectangles.

To make the glaze: Stir ketchup, vinegar, brown sugar, and Worcestershire sauce together in a small bowl and set aside.

Spray air fryer basket generously with cooking spray or line with parchment paper for easier clean up. Add meatloaves to basket and spray generously with cooking spray. Roast in the preheated air fryer until nicely browned, about 20 to 25 minutes, brushing evenly with ketchup mixture after 18 minutes of cooking. An instant-read thermometer inserted into the center should read at least 155 degrees F (68 degrees C).

Remove meatloaf from basket; let rest for 5 minutes. Garnish with thyme leaves and serve.