Don't ask me why Substack hides this, but for those--like me--who use RSS readers, this is what you want:
UPDATE: I spoke too soon. While that address will fetch the titles of items,
1) I can't mark them as read, and
2) the items don't appear to link to anything--i.e., if I double click on them they don't open in a browser tab/window.
NEW UPDATE: I just tried the substack feed in my RSS reader (Akregator) this morning and it's working perfectly now. As far as I can tell the restart did the trick.