Agreed - I would also like to add my support for this feature. It would be immensely helpful for a lot of us, and probably quite easy to implement given that indexes/contents pages already exist with this sort of functionality. The ability to add a footnote container into a Master, which would then update by itself to include any footnotes referenced on the given page, would be super useful; right now, I have to manually copy-paste the text frame out of the Master group, so that I can edit its height to fit a footnote container - not exactly neat/ideal.
My biggest issue with the manual approach is that I have to move the footnote text around depending on the text on the page - e.g. if I add an image, which pushes the footnote reference onto the next page, I then need to manually move the text frame/the note itself onto the next page, which is easy to forget and could lead to mistakes being published.