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.
* Initialize global loggers.
 *
 * Note that this is called very early in the process lifetime, so you should be
 * careful about what global state you rely on here.
 *
void InitLogging(const ArgsManager& args)
{
    init::SetLoggingOptions(args);
    init::LogPackageVersion();
}
namespace { 
int nMaxConnections;
int nUserMaxConnections;
int nFD;
ServiceFlags nLocalServices = ServiceFlags(NODE_NETWORK_LIMITED | NODE_WITNESS);
int64_t peer_connect_timeout;
std::set<BlockFilterType> g_enabled_filter_types;
} 
[[noreturn]] static void new_handler_terminate()
{
    
    
    
    
    std::set_new_handler(std::terminate);
    LogPrintf("Error: Out of memory. Terminating.\n");
    
    std::terminate();
};
bool AppInitBasicSetup(const ArgsManager& args, std::atomic<int>& exit_status)