Deliveries7d is how many deliveries SETTLED in the trailing 7 days — the attempts that ended ok or failed, so a delivery still retrying is in neither counter yet. It is counted from the log at read time rather than stored, and it is filled only on a list or a get; a create answers 0 because there is no history, which is why it is never omitted.
Failures7d is how many of those settled as failed — the subscriber never accepted it and no further attempt will be made. It is the numerator to Deliveries7d, over the same window.
Secret is the HMAC-SHA256 signing key a subscriber recomputes the signature with. It is returned exactly ONCE, on create: a later read of the endpoint omits it, so a lost secret is replaced rather than recovered.
Status is "active" or "disabled" — nothing else is accepted. A disabled endpoint keeps its subscription and receives no deliveries, except a manual test send, which goes out anyway.