This was quite interesting to look through! Perl 5.8.0 is over 20 years old ( https://dev.perl.org/perl5/news/2002/07/18/580ann/ ) while centOS 3.9 was released in 2007! At the same time it seems not-that-old and ancient. My personal anecdote with gnu parallel was running into it while working in academia. It worked well and saved me some time, but I felt that it was unreasonable of a tool to ask for a citation to pa…
It's a request, not a requirement. I see nothing wrong with the request nor if an individual decides to not cite it due to their principles/judgement.
But I still think making citation for gnu parallel is unreasonable. There is a huge body of software, of which gnu parallel is probably the least important, that contributed to (at least my) research. Blowing up citation lists with those makes the citation list borderline useless.
It makes citations into advertising space for software - it's bad enough being coerced to make it an advertisement for reviewers papers.