Add the carrots to a 5 or 6-quart slow cooker.
In the slow cooker, mix brown sugar, salt, cinnamon, and pepper. Stir to combine with the carrots.
Pour in the maple syrup and toss well to coat the carrots evenly.
Cut the butter into cubes or pats and spread them over the carrots.
Cover and cook on high for 3 hours, or on low for 5-6 hours, until the carrots are fork-tender. If possible, stir 1-2 times during cooking.
To thicken the glaze, choose one method:
Remove the lid for the final 30 minutes of cooking and switch to high.
Transfer carrots to a bowl, cover to keep warm. Pour liquid into a saucepan, simmer over medium-high heat, whisking until thickened (about 5 minutes).
If desired, garnish with fresh thyme before serving warm.