The "gallows" characters

[ReneZ]

This should be phrased more precisely of course. It means that any such plain text is necessarily in a language that exhibits the same features: special entropy, word patterns, replaceable characters.

While I cannot speak for the first of these characteristics, I would suggest that the other two are pretty normal for natural languages. Although the Voynich language my exhibit them more thoroughly than typical, I don't think they are peculiar.

As Jacques Guy used to say in the distant past, it will be hard to find any peculiarity that doesn't exist in some language somewhere.

On these two or three points, it may be a matter of taste whether one calls them peculiar or something more strongly.
If we take Mandarin Chinese for example, its rendition in Pinyin shows a structure that is even more rigid (I would say) than the Voynich MS. There are only some 400 allowed combinations (not counting the tones), and one can almost freely replace consonants by other ones and end up with another valid (pinyin) word.

Other East Asian languages will have various different rules.

For me the equivalent functions of k / t,  ch / sh, and l / r  remain highly suspicious. The best I can come up with is that someone was trying to render a language with unknown sounds, and had to make choices on the fly....

This doesn't work for everything though. The ending -m can appear where -l or -r can appear, but preferably at line ends. This more suggests an elaborate way of writing (say) -r,  but then there are labels that differ only by the ending -r or -m.
So far, I can only think of quite contrived explanations.