Multiple transpositions for Horns

I don't think we're talking about the same thing. What you describe is what happens when you switch between concert pitch and transposed pitch. What I suggest would make the transposition be determined by the clef applied by the user. In this proposal the transposition would be defined in instruments.xml based upon which clef is applied. So in the original conversation at https://musescore.org/en/node/311760 the F Horn would transpose down a 4th for a treble clef and up a 5th for a bass clef.

In pseudo code the definition would be something like
Treble clef: transpose down 4th
Bass clef: transpose up 5th

The default would be the first defined clef and all non mentioned clefs would follow the first definition.

BTW, this horn would be for people like me who transcribe older scores so the name listed in the instrument list might be "French Horn (traditional)" while the modern French horn definition would only define the treble clef and be called "French Horn"

Other instruments like the classical guitar would only need one definition because all classical guitar music is transposed the same way. The definition would make it so people could use either the treble or treble VB and the notes would still be correct on the staff.

It would also eliminate a lot of confusion that we see in the forums where people apply the wrong clef to an instrument, not realizing the definition requires a different clef. See https://musescore.org/en/node/311739 for a recent example. This would never have happened if the treble clef were defined as transposing the the treble VB were defined as not transposing.

I would keep your snippet from the definition to define the default clefs when the instrument is added to a score.

The point is to not need to create so many definitions for the same instrument. Bb tuba is another great example of an instrument that would benefit from this. In the case of the Horn, mid staff instrument changes are required because in the middle of a song it will switch from treble to bass clef (with a different transposition) then back, sometimes several times. If we simply define a treble and bass clef instrument (which is what I've done for myself) every time you change clef, a new mixer entry is created. I have a sound font and instrument definition that supports muted horn and horns in these score also play muted. These songs actually create 2 mixer entries per instrument change. The mid staff clef changes don't happen with Bb tubas but having one definition would lessen the chance of confusion if someone writing for a British Brass band selected the wrong Bb tuba and changed clefs. I've seen this happen.