That law, which was passed by the PA legislature with bi-partisan support (IOW, Republicans supported it), was declared unconstitutional by the state Court of Appeals, which was then reversed by the State Supreme Court, which held that the law WAS constitutional. So that means it was NOT illegal.
https://www.penncapital-star.com/campaigns-elections/pennsylvanias-mail-in-voting-law-survives-constitutional-challenge-by-gop-lawmakers/