I could apply the "redline" font attribute on text I added to an article, then apply the "strikeout" attribute on text I didn't want in the article. After that, I could save the article, save a copy under a new filename, and then run the aptly-named function on one of the copies to generate a clean copy with my edits. Does a similar function exist in Writer?
Yup. See "Tracking changes to a document" in Chapter 3 of the Writer Guide. To toggle on the facility, go to Edit | Changes > | Record.
It doesn't have to be those two attributes, ...
You will see what the default markings are; to change these, go to Tools | Options... | LibreOffice Writer | Changes. See Chapter 2 of the Writer Guide.
... but anything that would highlight the "should I stay?" and "should I go?" parts of the article and then easily generate a clean copy would be very helpful.
You can right-click any change in the text individually and choose Accept Change or Reject Change. Alternatively go to Edit | Changes >
Accept or Reject...; here you can accept or reject changes
individually, piecemeal, or globally.
Extra adulation if the markup attributes can be saved to .docx, which my main publisher uses exclusively.
I think you will find that is so - but sucking and seeing it is always best: experiment and send a trial document to your correspondent. But anyway, I don't see that you need this: surely you will keep your text documents in LibreOffice's native .odt format whilst you are editing and save a .docx version only of your clean copy after you have accepted all changes? If you want your correspondent to see the changes, another way might be to export a copy as PDF before accepting the changes.
I trust this helps.
Brian Barker