I have been thinking about this, and looking on web sites, and it seems to me I have been misunderstanding the meaning of 'derived work'.
From what I gather now from various web sites, including wikipedia, derived work means: "a new version of the original."
Thus, the monohtml docs are not a derived work. They're the result of using the original. Thus, no notice is necessary.
So, we don't need to remove them - at least not from old versions.
Does this make sense?
I think it is still a good idea to remove the file, and thus, to no longer build the monohtml going forward, because of the loopiness of not having permission to distribute the fo2html.xsl file and the requirement within Apache to distribute an src version with a release.
But it seems to me this issue can be closed.