Usually the Debian manual page author or package maintainer will send that upstream. Same goes for patches. Sometimes upstream doesn't want manual pages, or wants it in a different format, and the Debian person doesn't have time to rewrite it.
I also think that the idea that original authors must not accept manual pages is a way of explaining how the belief does not match reality, without accepting that it is the belief itself that is wrong. Certainly, the number of times that things work out like the net-tools example elsethread, where clearly the original authors do want manual pages, because they eventually wrote some, and end up duplicating Debian's (and FreeBSD's/NetBSD's) efforts, is evidence that contradicts the belief that there's some widespread no-manual-pages culture amongst developers.
I have sent about 50 or so patches upstream for the 300 packages I maintain and while it reduces the amount of work long-term it's also surprisingly amount of work.
Typically the Debian patches are licensed under the same license as the original project. So there is nothing stopping anyone who feels that more patches should be sent upstream to send them.
Typically the Debian maintai
If you're going to do that, then you should actually let people know. Otherwise don't do it. It's not about "but the license allows it", it's about what the right thing to do is.
Debian has given me the most grief of any Linux distro by far. Actually, Debian is the only system I can recall giving me grief. Debian pushes a lot of work to the broader ecosystem to people who never asked for it.
I didn't choose to be associated with Debian, but I have no choice in the matter. You did choose to be associated with the packages you maintain.
So don't give me any of that "but my unpaid time!". Either do the job properly or don't do it at all. Both are fine; no maintainer asked you to package anything. They're just asking you to not indirectly push work on them by shipping random (potentially broken and/or highly opinionated) patches they're never even told about.
Okay, I am hereby letting you know: Every single distro patches software. All of them. Debian, Arch, Fedora, Gentoo, NixOS, Alpine, Void, big, small, commercial, hobbyist. All of them.
And often it's not an unhelpful upstream, just an upstream that sees little use for man pages in their releases, and doesn't want to spend time maintaining documentation in parallel to what their README.md or --help provides (with which the man page must be kept in sync).