"However, in their eyes, and in mine at the time, this was a feature, not a drawback. No end-user computers are normally multiuser any more: people sharing computers was how things worked in the 1960 and 1970s, not in the 21st century."
That's mostly incorrect; any computer that's run by an organization does have multiple users. While each desktop is probably used only by one person, they have a restricted set of privileges because the administrators run some components. The administrators, while they don't use the machine routinely and may never access it physically, are effective users. Many organizational machines are also available for others to log in, even if they mostly don't do so. Home machines may still have multiple users if it is shared among family members or friends, which was a lot more common in the 1990s but is still common today.
A multi-user OS can have any number of users, including one. A single-user machine always has the limitations of that design. I think the decision by basically every OS to remove single-user limitations was necessary at the time and still remains useful.
And yes, servers are multi-user. Not in the sense that every client logs in and runs programs, but in the sense that there are multiple people who do things where they log in and run programs. The server's admin needs root privileges, whereas the clients who run a website from it probably don't. If there are multiple admins, they probably each have their own user account which enables per-user privilege management (even if it's only locking out a user when they leave). Not every server has one admin or a set who can all act as root.