I think Why did Shostakovich not release his 4th, but did release his 5th symphony? is a question for Music Fans. Some people do think it's a good fit here. I think this is equivalent to "Why didn't Some Band ever release the songs intended for the record Watchamacallit", which would be shot down in flames here. What is the difference, except the age of the music.
We are, in fact, in very interesting territory here. There is a good argument for suggesting that a historical question like this is better suited to the proposed Music Fans site. But musical analysis and musicology are not restricted to the score (or recording); they don't take place in a vacuum. It is perfectly valid, and in many respects desirable, for analysis of a piece of music to be informed (and enhanced!) by a historical and/or sociological perspective.
However, it is true that music from different eras and traditions does lend itself more readily to different analytical and musicological approaches. "Classical" music (or "composed" or "art" music) is readily analysed by means of the written score; popular and folk musics often make greater use of related "extra-musical" information when analysed.
But I would point out that any kind of music is open to different analytical approaches: popular music can be analysed using the "notes-on-the-page" (even if this is the result of transcription); "classical" music can be analysed taking into account a historical/sociological perspective. Crucially, I would say that whatever the analytical or musicological approach, all eras and traditions of music are equally valid, and should be treated as such on this site. All existing music is part of music history.
This does, though, require the Music.SE community to judge as a whole which questions and answers genuinely explore an interesting aspect of a piece of music. I don't think specific eras, types of music, artists or composers can be used to judge what is on or off topic.
I would say again what I said on my comment: I think a question like "Concerning Beethoven's 9th ..." would fit here, but a question like "Who is Justin Bieber referring to in 'Baby'?" would not.
Like you, I'm not 100% sure what eras are being concerned in Music History, but I believe that musicians that actually changed the music as people then knew it, can be considered on topic on the history.
Era should not be a factor when trying to decide if a question is on or off topic. Content should be as is with everything else on the site.
I think the bigger problem is there has not really been a lot of history questions asked on the site so what is on or off topic in this area seems to be somewhat unknown. For example, looking though the 71 history tagged questions we have I found this question "Vivaldi's “The Four Seasons”, Who Chose the Title?" which while historical seems more fitting on a fan site then here and in my opinion probably should have been closed. I also found another question which is more about modern music "What is the history of considering Rock 'n' Roll to be “the Devil's music”?" which seems right at home here in my opinion.
I think in general we should discuss what qualifies as a history here especial when the fan site goes into beta, but I don't think we should judge based on era.
Just a suggestion: Perhaps a line can be drawn between "Songs" and "Works"/"Opus".
Rebecca Black - Friday is a clearly just a song.
Shostakovich - Opus 43, 4th Symphony is a work.
Given that Rebecca Black up until this point, is not referring to her songs as, Opus 13 - Friday, this is probably a pretty decent line to draw in the sand.