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.
block_notify_genesis_wait_connection = uiInterface.NotifyBlockTip_connect(std::bind(BlockNotifyGenesisWait, std::placeholders::_2));
    } else {
        fHaveGenesis = true;
    }
#if HAVE_SYSTEM
    const std::string block_notify = args.GetArg("-blocknotify", "");
    if (!block_notify.empty()) {
        uiInterface.NotifyBlockTip_connect([block_notify](SynchronizationState sync_state, const CBlockIndex* pBlockIndex) {
            if (sync_state != SynchronizationState::POST_INIT || !pBlockIndex) return;
            std::string command = block_notify;
            ReplaceAll(command, "%s", pBlockIndex->GetBlockHash().GetHex());
            std::thread t(runCommand, command);
            t.detach(); 
        });
    }
#endif
    std::vector<fs::path> vImportFiles;
    for (const std::string& strFile : args.GetArgs("-loadblock")) {
        vImportFiles.push_back(fs::PathFromString(strFile));
    }
    chainman.m_load_block = std::thread(&util::TraceThread, "loadblk", [=, &chainman, &args] {
        ThreadImport(chainman, vImportFiles, ShouldPersistMempool(args) ? MempoolPath(args) : fs::path{});
    });
     Wait for genesis block to be processed
    {
        WAIT_LOCK(g_genesis_wait_mutex, lock);
         We pr
         Threa
         check 
        while (!fHaveGenesis && !ShutdownRequested()) {