I had found a meta discussion from 2010, but I imagine policy may have changed since then?
I have a couple of questions I would like to ask regarding meter, repetition, general structure, etc. Is (the lyrical part of) songwriting on-topic here?
If questions are specifically about lyrics and not music, they're fine. Questions purely about music can be asked on Music|SE.
However, songs aren't just music and lyrics; there's a lot of interplay between the two. Questions that mention the music but are primarily oriented to the lyrics are, IMO, fine.
If you are asking about writing song lyrics, yes. If you are asking about someone else's lyrics then the same rules would apply as to someone else's work. If you are asking "are these lyrics good" or "please write lyrics" then it's off-topic.
Basically, if it would be on-topic if it was a book instead of a song then it's on topic.