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.
if (args.GetBoolArg("-reindex-chainstate", false)) {
            return InitError(_("Prune mode is incompatible with -reindex-chainstate. Use full -reindex instead."));
        }
    }
    
    if (args.GetBoolArg("-forcednsseed", DEFAULT_FORCEDNSSEED) && !args.GetBoolArg("-dnsseed", DEFAULT_DNSSEED)){
        return InitError(_("Cannot set -forcednsseed to true when setting -dnsseed to false."));
    }
    
    size_t nUserBind = args.GetArgs("-bind").size() + args.GetArgs("-whitebind").size();
    if (nUserBind != 0 && !args.GetBoolArg("-listen", DEFAULT_LISTEN)) {
        return InitError(Untranslated("Cannot set -bind or -whitebind together with -listen=0"));
    }
    
    if (!args.GetBoolArg("-listen", DEFAULT_LISTEN) && args.GetBoolArg("-listenonion", DEFAULT_LISTEN_ONION)) {
        return InitError(Untranslated("Cannot set -listen=0 together with -listenonion=1"));
    }
    
    int nBind = std::max(nUserBind, size_t(1));
    nUserMaxConnections = args.GetIntArg("-maxconnections", DEFAULT_MAX_PEER_CONNECTIONS);
    nMaxConnections = std::max(nUserMaxConnections, 0);
    nFD = RaiseFileDescriptorLimit(nMaxConnections + MIN_CORE_FILEDESCRIPTORS + MAX_ADDNODE_CONNECTIONS + nBind + NUM_FDS_MESSAGE_CAPTURE);
#ifdef USE_POLL
    int fd_max = nFD;
#else
    int fd_max = FD_SETSIZE;
#endif
    
    
    nMaxConnections = std::max(std::min<int>(nMaxConnections, fd_max - nBind - MIN_CORE_FILEDESCRIPTORS - MAX_ADDNODE_CONNECTIONS - NUM_FDS_MESSAGE_CAPTURE), 0);