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.
{
    return AbsPathForConfigVal(args, args.GetPathArg("-pid", BITCOIN_PID_FILENAME));
}
[[nodiscard]] static bool CreatePidFile(const ArgsManager& args)
{
    std::ofstream file{GetPidFile(args)};
    if (file) {
#ifdef WIN32
        tfm::format(file, "%d\n", GetCurrentProcessId());
#else
        tfm::format(file, "%d\n", getpid());
#endif
        return true;
    } else {
        return InitError(strprintf(_("Unable to create the PID file '%s': %s"), fs::PathToString(GetPidFile(args)), SysErrorString(errno)));
    }
}
#if HAVE_SYSTEM
static void ShutdownNotify(const ArgsManager& args)
{
    std::vector<std::thread> threads;
    for (const auto& cmd : args.GetArgs("-shutdownnotify")) {
        threads.emplace_back(runCommand, cmd);
    }
    for (auto& t : threads) {
        t.join();
    }
}
#endif
void Interrupt(NodeContext& node)