Place chicken breasts in the bottom of the Instant Pot. Put all remaining chicken ingredients on top of chicken in the Instant Pot.
Place lid on Instant Pot. Press MANUAL or PRESSURE COOK button and set timer for 12 minutes (if chicken breasts are frozen, set timer for 22 minutes).
Once cooking time is complete, switch the valve on top to “venting” (i.e. do a quick pressure release).
Remove chicken breasts from the Instant pot and shred using two forks. Add shredded chicken back into the pot and mix with sauce in the pot until all the chicken is covered.
Prepare the coleslaw by combining the coleslaw mix, purple cabbage, and shredded carrot in a large bowl. In a smaller bowl, mix mayonnaise, vinegar, celery seed, and sugar together.
Toss dressing with cabbage mix until cabbage is covered in dressing. Season with salt and pepper.
Serve each taco with a tortilla topped with shredded chicken, coleslaw, fresh pineapple, cilantro, and onion slices.