> GNU parallel wants to treat it like a requirement of using the software

I have never felt this, and that is not how FOSS works. By definition, they cannot restrict how you use the software. Thus, the citation request is just a request. Hypothetically, you could slander and ruin the author's life (the extreme polar opposite of a citation) and still freely use the software.

This is no different than an author asking users to retweet, post on reddit, etc. Certainly it may be annoying to some, but it does not restrict how you may use or fork the software.

One could fork GNU parallel to remove the copyright, and let the democratic public user base vote on whether they care enough to use your fork, or if they think you (or the other author) are an asshole, etc.