See also: GNU parallel showing a nag for being cited in papers.
https://git.savannah.gnu.org/cgit/parallel.git/tree/src/para...
https://git.savannah.gnu.org/cgit/parallel.git/tree/src/para...
From https://www.gnu.org/software/parallel/parallel_design.html#C...:
> As the request for citation is not a legal requirement this is acceptable under GPLv3 and cleared with Richard M. Stallman himself. Thus it does not fall under this: https://www.gnu.org/licenses/gpl-faq.en.html#RequireCitation