Well, for those who are interested, I've added some information to <https://wiki.debian.org/NewInBuster>. I'm trusting that the ALWAYS_SET_PATH thing from that random web page was actually correct, because verification would take a lot of work. It's a wiki, so someone else can correct it if it's wrong.
When I linked to EnvironmentVariables I also found a section at the end of *that* page which describes the current behavior of su, so I updated that page slightly as well. I can't even begin to guess how many places assume the current su behavior or how difficult it will be to find and change them all.
