Yes, Pennsylvania is one of the few states that does not follow the federal rules on 401(k) contributions. PA does NOT exclude 401(k) contributions from a taxpayers income - thus, making contributions taxable. This applies even if you are a PA resident working in DE - you will have to add back any contributions to a 401(k) for PA return purposes (if your payroll company hasn't already done this for you).
https://revenue-pa.custhelp.com/app/answers/detail/a_id/273/~/are-my-contributions-to-a-401%28k%29-p...
