There is
Tarski went further and showed that some propositions cannot even be said to be true or false!!!
in A tutorial by Mike Nahas ( https://coq.inria.fr/tutorial-nahas ).
But this sentence is somehow vague and probably is NOT really right, see:
https://math.stackexchange.com/questions/2693922/how-did-tarski-show-that-some-propositions-are-neither-true-nor-false
Would the expert Mike Nahas like to improve the tutorial?
I don’t think Mike Nahas has claimed to be an expert, just someone who wanted to help by providing a short introduction to Coq.
As you can see in the postscript part of the tutorial, you may reach the author by e-mail to inform him that this sentence is unclear (very interesting reference by the way!):
Contact
HTML and Coq versions of the tutorial are hosted on the author’s professional website. https://mdnahas.github.io/doc/nahas_tutorial.html https://mdnahas.github.io/doc/nahas_tutorial.v The author, Michael Nahas, can be reached at michael@nahas.com
And for what is worth, here is a direct link to the sources of this tutorial: https://github.com/mdnahas/mdnahas.github.io/blob/master/doc/nahas_tutorial.v