@Zanna This is very slow-going, partly because I'm doing it in very short bursts, but partly because of writer's block... which is not something I usually experience in connection with writing questions and answers for Stack Exchange. (Sometimes editing of old, highly-upvoted posts, where I feel constrained in changing the meaning, but not when writing them originally.)

Anyway, here's the current draft of the answer, which is the same as it was before with some rewordings, a number of links, and one additional paragraph:

For years, Ubuntu has [shipped a patched version of `sudo` that preserves `$HOME` by default][1]. Besides Ubuntu and its derivatives, [very few other operating systems (perhaps no others) do this][2]. It has been [**decided that this causes more problems than it solves**][3], and [starting in Ubuntu 19.10][4], `$HOME` is no longer one of the few environment variables `sudo` preserves.

In terms of *what* the change is and how it affects users, the key points are:

- **As of Ubuntu 19.10, <code>sudo *command*</code> does what <code>sudo -H *command*</code> does in previous releases.** It can…