Apologies for necro'ing the thread. We updated the subscriber agreement today and I found this thread looking for guidance on this exact issue.
Is there a way to know the URL for the current subscriber agreement before registering a user? For example, at the /directory endpoint.
You can get the current subscriber agreement URL for Let's Encrypt from the /terms endpoint. E.g.:
curl -I https://acme-v01.api.letsencrypt.org/terms 2>/dev/null | grep "Location:"
Location: https://letsencrypt.org/documents/LE-SA-v1.1.1-August-1-2016.pdf
The ACME Draft also suggests an optional method for servers to advertise their ToS location as part of a /directory metadata entry. Let's Encrypt does not currently implement the metadata entry described in the draft because it will break some clients. We will likely support it when we next make a breaking change.