Voynichese is a numeric cipher?

[Jorge_Stolfi]

For example, we can imagine a hypothetical scenario where someone decides to create an extremely difficult to break cipher, and write a book in the ciphertext. The motivation could simply be to show others "hey look, I encrypted a whole book! I guarantee you can't decrypt it! It's surely impossible".

There are in fact notable historical examples of that.  I recall one from the 15th century (give or take a century) that was cracked only a couple decades ago.  Another one was reportedly written by Raphael Mnishovsky, who is to blame for the "Bacon" meme and was (or wanted to be) a professional cryptographer. But I don't know whether the latter survived.

Still, I don't think that such a "demo" book would waste so much work and vellum on all those plant drawings and cosmo diagrams.

All the best, --stolfi