I recently asked a question about how to do something in LilyPond on SO here but got downvoted and another user (not the down-voter) pointed out that it is not a programming question. But it is. I've asked too on the Music SE here but I am confused. Where should such a question live on SE? Is it OK to cross-post to two relevant SE's? Why is LilyPond frowned on in SO?
LilyPond is very on topic here as it is a music engraving program like Finale of Silibus, just the difference is that the output is generated via text file instead of GUI interface. The description on the website describes it as:
LilyPond is a music engraving program, devoted to producing the highest-quality sheet music possible.
IMO it makes much more sense to ask it here as the audience here not only is familiar with LilyPond, but is familiar with what sheet music should look like and the basic rules of music. One stat you can look at that backs this up is even though Music SE has much lower traffic and questions than SO, it has two more questions tagged with LilyPond here shows that this community overall has a better space carved out for it.
LilyPond questions also typically require some knowledge of the notation as that can be the reason for the errors in the output as seen in this question.
If the question is about how to get lilypond to produce the musical notation that you want, then it's on topic here, and likely to get better/quicker answers (though it is still the case the mailing list is where you'll find the best lilypond support). That being said, I can envision some types of questions where you are getting into the scheme coding side of things (maybe in support of automated score content generation) where it really is a coding question. In those cases, I'd think it belongs on SO, even if they end up closing it.
I wonder whether it might have been better to have a dedicated Music Notation stackexchange site? Within this site questions could be all focussed on notation and engraving in formats such as Lilypond, rather than mixed in with questions about practice and theory.