Are only questions about modern Russian on-topic? Or are the questions about Old Russian and Russian language dialects also welcomed on this site?
Old Russian is still Russian, only old :)
Church Slavonic is not Russian (and has never been, it's old Bulgarian), but heavily influenced Russian and is heavily influenced by Russian, so I think for a lack of better place the questions about it should be asked here.
But which is most important, the same experts usually express interest to all three subjects.