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.
const ChainType chain_type,
        const std::vector<const char*>& extra_args,
        const bool coins_db_in_memory,
        const bool block_tree_db_in_memory)
    : TestingSetup{ChainType::REGTEST, extra_args, coins_db_in_memory, block_tree_db_in_memory}
{
    SetMockTime(1598887952);
    constexpr std::array<unsigned char, 32> vchKey = {
        {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1}};
    coinbaseKey.Set(vchKey.begin(), vchKey.end(), true);
    
    this->mineBlocks(COINBASE_MATURITY);
    {
        LOCK(::cs_main);
        assert(
            m_node.chainman->ActiveChain().Tip()->GetBlockHash().ToString() ==
            "571d80a9967ae599cec0448b0b0ba1cfb606f584d8069bd7166b86854ba7a191");
    }
}
void TestChain100Setup::mineBlocks(int num_blocks)
{
    CScript scriptPubKey = CScript() << ToByteVector(coinbaseKey.GetPubKey()) << OP_CHECKSIG;
    for (int i = 0; i < num_blocks; i++) {
        std::vector<CMutableTransaction> noTxns;
        CBlock b = CreateAndProcessBlock(noTxns, scriptPubKey);
        SetMockTime(GetTime() + 1);
        m_coinbase_txns.push_back(b.vtx[0]);
    }
}
CBlock TestChain100Setup::CreateBlock(