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.
{
    try {
        LOCK(m_cs_fee_estimator);
        int nVersionRequired, nVersionThatWrote;
        filein >> nVersionRequired >> nVersionThatWrote;
        if (nVersionRequired > CLIENT_VERSION) {
            throw std::runtime_error(strprintf("up-version (%d) fee estimate file", nVersionRequired));
        }
        
        
        unsigned int nFileBestSeenHeight;
        filein >> nFileBestSeenHeight;
        if (nVersionRequired < 149900) {
            LogPrintf("%s: incompatible old fee estimation data (non-fatal). Version: %d\n", __func__, nVersionRequired);
        } else { 
            unsigned int nFileHistoricalFirst, nFileHistoricalBest;
            filein >> nFileHistoricalFirst >> nFileHistoricalBest;
            if (nFileHistoricalFirst > nFileHistoricalBest || nFileHistoricalBest > nFileBestSeenHeight) {
                throw std::runtime_error("Corrupt estimates file. Historical block range for estimates is invalid");
            }
            std::vector<double> fileBuckets;
            filein >> Using<VectorFormatter<EncodedDoubleFormatter>>(fileBuckets);
            size_t numBuckets = fileBuckets.size();
            if (numBuckets <= 1 || numBuckets > 1000) {
                throw std::runtime_error("Corrupt estimates file. Must have between 2 and 1000 feerate buckets");