Interchangeable ch/sh and k/t characters

[ReneZ]

Character entropy is totally dependent on the encoding of phonemes.  The character entropy of Italian would increase noticeably if one replaced "ch"->"k", "gn"->"ñ", "gl"->"ł', "sc"->"š", etc.  Even more if one replaced every open "e" by "ɛ", every open "o" by "ɔ", and marked every stressed vowel with a diacritic.

A couple of observations related to that...
First, the dependency is likely to be less than one might expect. It is certainly a lot less than the difference between Italian and Voynichese in any of the common transliteration alphabets.
Secondly, making such substitutions means it is no longer Italian.
Thirdly, the fact that such changes show up in the character entropies (single char- and bigrams) make this a useful measure IMHO. 

Word entropy is less dependent on the encoding.  But even that is affected by orthography, e.g. by word splitting and joining.  The word entropy of Italian would be lower if oblique pronouns were split from the verb ("ditemelo" -> "dite me lo") and compounds were split into components ("automoble" -> "auto mobile", "solamente" -> "sola mente", etc.  And it is lower also if the text is heavily abbreviated or in shorthand, so that many word types are merged ("pasto","pesto","posta" -> "pst" etc.)
Anyway, IIRC the word entropy of Voynichese was about 10 bits per word, which was well within the range of European languages.

Word entropy can barely be measured for the Voynich MS text. Word pair entropy is completely out of reach. For single-word entropy, the uncertainty of the alphabet, the handwriting itself and the word spacing makes that this cannot be estimated reliably. As an example, one will get a highly variable number of hapax from the text, depending on which transliteration (of the same text) one uses.
There are some figures related to that in my 2022 conference paper.