Confirm chicken is fully thawed and note any retained salt solution. Pat dry and trim; halve an unusually thick breast horizontally.
Spread 1/2 cup salsa over the bottom of a 4- to 6-quart slow cooker.
Arrange chicken in one layer. Coat evenly with taco seasoning and spoon remaining salsa over and around it.
Cover and cook on high for 2 to 3 hours, beginning checks at 2 hours.
Remove each breast when its thickest center reaches 165°F. Rest on a clean plate for 5 minutes.
Shred into medium strands. Reduce watery sauce separately before returning chicken.
Fold chicken into enough hot sauce to coat, taste and serve promptly.
