Yes it's systemd. Thanks for explanation. I thought about modifying the .service file, too, but IMO it's an 'ugly hack', editing squid.conf is more beautiful.
As you ask for ideas to warn other users: I took a look in the changelog and found systemd to be supported in squid 4 and I also would have looked at /usr/share/doc/squid/README. Perhaps something like apt-listchanges would be nice, but for a CLI based proxy server I'm not really shure. For buster it's a good idea to say some words in the release notes. Thank you, Christian
