>>I rather imagine that there are still rules, at least in
principle, about ones parish.<<
What rules?
Where?
Maybe it’s a regional thing. I lived in a suburb of Cleveland called Brook Park. Our nearest parish was SS Peter and Paul, up the street. We belonged to a parish two suburbs over in Berea OH, because my Great Uncle was the pastor. It was a Polish Parish.
I’m not really sure where you are getting your ideas about rules. There are NO rules about where you have to belong. Seriously. If you can find any, post them. Cause I have been at parishes all my life. I’ve lived in different cities. People in my area North of Detroit drive into the city and belong to parishes there. Others drive to my area because my current parish is very traditional.
Show me the rules. Cause with a Parish Priest and a Bishop in my family, I’ve never heard of them.
Catholic "law" is very messy in the eyes of Protestants because it's all about a grand principle -- but with numerous exceptions. This is foreign to English Common Law on which our legal system is based, so there are lots of opportunities for misunderstanding.