After conversation on irc it appears there's some cases where Bitcoin core will serve feerate information even though it loaded a fairly old fee estimates file and is still syncing the chain. This is pretty dangerous for lightning nodes, at least on legacy channels or pre-package-relay.

The immediate improvement would be to store fee estimates to disk once an hour or so to reduce the chance of having an old file. From there this case could probably be detected and refuse to serve estimates until we sync.
void Write(AutoFile& fileout) const;
    /**
     * Read saved state of estimation data from a file and replace all internal data structures and
     * variables with this state.
     */
    void Read(AutoFile& filein, int nFileVersion, size_t numBuckets);
};
TxConfirmStats::TxConfirmStats(const std::vector<double>& defaultBuckets,
                                const std::map<double, unsigned int>& defaultBucketMap,
                               unsigned int maxPeriods, double _decay, unsigned int _scale)
    : buckets(defaultBuckets), bucketMap(defaultBucketMap), decay(_decay), scale(_scale)
{
    assert(_scale != 0 && "_scale must be non-zero");
    confAvg.resize(maxPeriods);
    failAvg.resize(maxPeriods);
    for (unsigned int i = 0; i < maxPeriods; i++) {
        confAvg[i].resize(buckets.size());
        failAvg[i].resize(buckets.size());
    }
    txCtAvg.resize(buckets.size());
    m_feerate_avg.resize(buckets.size());
    resizeInMemoryCounters(buckets.size());
}
void TxConfirmStats::resizeInMemoryCounters(size_t newbuckets) {