Does anyone still use a line editor these days?

Yes. I use a line editor but an advanced one. I'd prefer emacs but tend to us vim because it works better over poor Internet connectivity. I can (and have) used ed, nano, pico, and the like, but find that a little context helps.

The mental model needed for line editing and the one needed for visual editing is different. The former requires a mental model of the document being edited where the latter just requires that you can read. There is a world of difference.

Like UNIX's or CP/M's Ed, or DOS's Edlin?

Thank goodness no, well certainly I don't. In fact, I've not seen anyone I know in IT using a line editor for years. Presumably those who use FreeDOS etc. still do.

Even in the days when there was nothing else to use I always hated them especially especially so CP/M's Ed. Moreover, its typewriter-like manual (which I likely still have somewhere), was horrible. But unlike much software these days, at least Ed had a manual!