Place the chicken breasts in the slow cooker.
Add salsa, ground cumin, chili powder, corn, black beans, and low-sodium chicken broth to the slow cooker and stir to combine.
Cook on low for 8 hours or on high for 4 hours, until the chicken is very tender.
Remove the chicken and shred it with two forks; return the shredded chicken to the slow cooker and stir. Continue cooking on low for 30 minutes (or on high for 15 minutes) to let flavors meld.
Warm the whole-wheat flour tortillas if desired. Spoon about 2 tablespoons of the shredded chicken mixture into each tortilla.
Top tacos with shredded romaine, diced tomato, shredded cheddar, diced avocado, and a dollop of plain Greek yogurt. Serve immediately.