final boolean deferringUpgrade(Node node, Instant instant) {
        return deferOsUpgrade.with(node.cloudAccount(), nodeRepository.zone()).value() ||
                !node.status().osVersion().downgrading() && // Never defer downgrades
                node.history().age(instant).compareTo(gracePeriod()) <= 0;
    }
Returns whether node is deferring upgrade at given instant
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/os/OsUpgrader.java