Can this be made without a slow cooker? How?

You certainly could do this recipe on the stovetop, poaching the chicken in a covered pan over very low heat. You could also do it in a very low oven, perhaps set at 275F or so. The method is still poaching in milk, at low temps- regardless of the implement you chose to provide the heat. Slow cookers are just better able to maintain a steady low heat and require less attention from you to do the job. If you do it on the stove top, you will need to hang about to keep tabs on the pan to ensure it doesn't do a full boil. That's less trouble with an oven, but some ovens get finicky at low temps.