It's used by academics to write conference papers, it's used for most of the Racket books for both Web and camera-ready typesetting for print, and I've even used it for embedded API docs.

https://docs.racket-lang.org/scribble/

(That manual itself was written in Scribble.)

(I'm actually thinking of moving from Scribble to Markdown for embedded API docs, as part of an open source ecosystem goal, to make docs for reusable modules more lightweight to add, but it's hard to give up some things about Scribble. For example, for one module, I needed substantial documentation about each opcode, and wanted it to be formatted similarly to an API function, but with different meta properties, and some tricky formatting. Scribble let me make a simple semantic form for that, separate from formatting, like LaTeX would.)