[itsme]

I agree with Sciurius that transposing should allow all keys. The additional effort to skip those "theoretical keys" is not important compared with the effort to think about why a certain key is missing.
I disagree with "chordpro songs don't have a key signature, just chords". Songs do have a key signature (at least this kind of songs that Sciurius and me usually play) and this is why the setting "Detect Key By" First Chord / Last Chord / Chord Progression and the additional possibility to overwrite it using {key:x} absolutely makes sense.
I play a lot of waltzes which change the key, so the meta data in my library says e.g. Keys: Am, Em, G wheras the key signature is Am. Or in some cases "First Chord" or "Last Chord" go wrong, e.g. "Night And Day" by Cole Porter in the key of C starts with an AbMaj7 and many tangos end with a V7 chord.