The Integration Guide suggests subscribing to the API Announcements category on this forum. There's also a Technical Updates mailing list which a Let's Encrypt staff member recommends as the preferred method. They generally have the same things posted to them, in my experience, though I think they may be trying to move people towards the mailing lists in general.
But most well-set-up systems shouldn't care when the intermediates change; if you post more about why your system does people may have suggestions on possible improvements.