Season chicken with salt and pepper. Arrange carrots and onion in a 6-quart slow cooker.
Arrange chicken in one layer over vegetables.
Whisk maple syrup, mustard, vinegar, Worcestershire, and thyme; pour evenly over chicken.
Cover; cook on low for 3 1/2 to 4 hours until thighs reach 175°F and carrots are tender.
Transfer chicken and vegetables to a platter; switch cooker to high.
Whisk cornstarch with cold water; stir into sauce. Cook uncovered for 8 to 10 minutes until lightly thickened.
Spoon maple sauce over chicken and carrots.