If you read the FAQ they have regarding the citation notice for GNU Parallel, it's made clear that it is not part of the license in any way and only applies to projects that are part of/the basis for academic papers. If it does apply to your project and you don't cite, at absolute worst you could get in trouble with your university or the academic community but even then it's almost certainly going to be mild at worst.
But importantly you can use the software however you want that is compatible with GPLv3. That includes ignoring or removing the citation notice without paying a cent. However just because it's legal doesn't mean it won't come with the potential for social consequences.
== Is the citation notice compatible with GPLv3? ==
Yes. The wording has been cleared by Richard M. Stallman to be compatible with GPLv3. This is because the citation notice is not part of the license, but part of academic tradition.
Therefore the notice is not adding a term that would require citation as mentioned on:
https://www.gnu.org/licenses/gpl-faq.en.html#RequireCitation
The link only addresses the license and copyright law. It does not address academic tradition, and the citation notice only refers to academic tradition.
[...]
and from the GPL faq itself (which said citation FAQ references):
Does the GPL allow me to add terms that would require citation or acknowledgment in research papers which use the GPL-covered software or its output? (#RequireCitation)
No, this is not permitted under the terms of the GPL. While we recognize that proper citation is an important part of academic publications, citation cannot be added as an additional requirement to the GPL. Requiring citation in research papers which made use of GPLed software goes beyond what would be an acceptable additional requirement under section 7(b) of GPLv3, and therefore would be considered an additional restriction under Section 7 of the GPL. And copyright law does not allow you to place such a requirement on the output of software, regardless of whether it is licensed under the terms of the GPL or some other license.
TLDR: The citation notice is a "cite it in academic works or pay me" agreement that is as legally binding as a pinky promise. You can break it without concern but some people may look negatively on that and it may come with social consequences.
But importantly you can use the software however you want that is compatible with GPLv3. That includes ignoring or removing the citation notice without paying a cent. However just because it's legal doesn't mean it won't come with the potential for social consequences.
https://git.savannah.gnu.org/cgit/parallel.git/tree/doc/cita...and from the GPL faq itself (which said citation FAQ references):
https://www.gnu.org/licenses/gpl-faq.en.html#RequireCitation---
TLDR: The citation notice is a "cite it in academic works or pay me" agreement that is as legally binding as a pinky promise. You can break it without concern but some people may look negatively on that and it may come with social consequences.