There is a feature in MuseScore that allows you to change the head types of notes. A quarter note - in the same Voice, eg 1 - can appear as a half note or in whole note (by hidding here the stem). With four available voices, this allows you all kinds of things, as you wish.
So, if Fernando Sor lived nowadays, and he would like to add a fifth voice, he could do it as well (the displaying of the second measure - image below - is better, no one now would show the E bass whole note displayed in the middle of the measure)
And for Kitharologus, Exercice 14, it's still the same principle. I changed some heads of note, I added some symbols of rests when necessary. With some skills with the program, of course, it's really not a big deal to achieve this.
If all six voices were implemented, I would have saved a bit time, I agree. But I think we can say that they are corner cases, unusual. And in terms of implementation, the human cost and time would probably be considerable (I am not a programmer, I can be wrong) for cases of compatibility with other versions, other programs and formats (musicxml, Midi, GP, and others).
In addition, the guitar is really a case apart, a polyphonic instrument that is written on a single staff (that's perhaps why we cherish it so much!)
As written already, we can use two staves if necessary (it's not so uncommun in the contemporary repertoire)
The piano and others play on two or three staves, and the melodic instruments (the orchestra instruments eg) are not impacted, of course.
Composers would probably be the most concerned. But again, it's possible to show just about anything you want and how you want with MuseScore.
And as for the possibility of applying these 5 or 6 voices in a polyphonic language, I don't know how, even the greatest virtuoso, would be be able to conduct and hold several voices at once (already, currently, with 4 voices, we are forced to make a choice, to shorten such or such note duration, because simply the left hand can not do it.)
Finally, as I said before, this feature request is not illegitimate in itself. But if you want it, you have to ask it - nobody will do it for you - and defend it and argue solidly.
Out of curiosity, I looked at the number of official feature requests currently in MuseScore. There are 31 pages! For a staggering total of 767 !! It makes you dizzy!
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