Post 1983, you are correct that "illegitimacy" does not have any meaning in canon law.
I was told this by a priest at St. Patrick’s in 1977. He never mentioned the priesthood business. (I had to take lessons when I went back to the Catholic church, since I was brought up pre-Vatican II.)
There have always been illegitimate babies but it’s still merely a civil category. Illegitimacy as an impediment to the priesthood was merely a practical consideration on the theory that if the parents couldn’t control themselves, there was a good chance the illegitimate son wouldn’t be able to either — at least that’s how they explained when I was in high school. I’m not sure it was an absolute impediment or if individual cases could be exceptions.