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.
int nHashType = SIGHASH_ALL;
    std::map<int, bilingual_str> input_errors;
    assert(SignTransaction(mempool_txn, &keystore, input_coins, nHashType, input_errors));
    
    if (submit) {
        LOCK(cs_main);
        const MempoolAcceptResult result = m_node.chainman->ProcessTransaction(MakeTransactionRef(mempool_txn));
        assert(result.m_result_type == MempoolAcceptResult::ResultType::VALID);
    }
    return mempool_txn;
}
std::vector<CTransactionRef> TestChain100Setup::PopulateMempool(FastRandomContext& det_rand, size_t num_transactions, bool submit)
{
    std::vector<CTransactionRef> mempool_transactions;
    std::deque<std::pair<COutPoint, CAmount>> unspent_prevouts;
    std::transform(m_coinbase_txns.begin(), m_coinbase_txns.end(), std::back_inserter(unspent_prevouts),
        [](const auto& tx){ return std::make_pair(COutPoint(tx->GetHash(), 0), tx->vout[0].nValue); });
    while (num_transactions > 0 && !unspent_prevouts.empty()) {
        
        CMutableTransaction mtx = CMutableTransaction();
        const size_t num_inputs = det_rand.randrange(24) + 1;
        CAmount total_in{0};
        for (size_t n{0}; n < num_inputs; ++n) {
            if (unspent_prevouts.empty()) break;
            const auto& [prevout, amount] = unspent_prevouts.front();
            mtx.vin.push_back(CTxIn(prevout, CScript()));
            total_in += amount;
            unspent_prevouts.pop_front();
        }