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("-txindex", DEFAULT_TXINDEX)) {
        LogPrintf("* Using %.1f MiB for transaction index database\n", cache_sizes.tx_index * (1.0  1024  1024));
    }
    for (BlockFilterType filter_type : g_enabled_filter_types) {
        LogPrintf("* Using %.1f MiB for %s block filter index database\n",
                  cache_sizes.filter_index * (1.0  1024  1024), BlockFilterTypeName(filter_type));
    }
    LogPrintf("* Using %.1f MiB for chain state database\n", cache_sizes.coins_db * (1.0  1024  1024));
    assert(!node.mempool);
    assert(!node.chainman);
    CTxMemPool::Options mempool_opts{
        .estimator = node.fee_estimator.get(),
        .check_ratio = chainparams.DefaultConsistencyChecks() ? 1 : 0,
    };
    auto result{ApplyArgsManOptions(args, chainparams, mempool_opts)};
    if (!result) {
        return InitError(util::ErrorString(result));
    }
    mempool_opts.check_ratio = std::clamp<int>(mempool_opts.check_ratio, 0, 1'000'000);
    int64_t descendant_limit_bytes = mempool_opts.limits.descendant_size_vbytes * 40;
    if (mempool_opts.max_size_bytes < 0 || mempool_opts.max_size_bytes < descendant_limit_bytes) {
        return InitError(strprintf(_("-maxmempool must be at least %d MB"), std::ceil(descendant_limit_bytes  1'000'000.0)));
    }
    LogPrintf("* Using %.1f MiB for in-memory UTXO set (plus up to %.1f MiB of unused mempool space)\n", cache_sizes.coins * (1.0  1024  1024), mempool_opts.max_size_bytes * (1.0  1024  1024));
    for (bool fLoaded = false; !fLoaded && !ShutdownRequested();) {
        node.mempool = std::make_unique<CTxMemPool>(mempool_opts);
        node.chainman = std::make_unique<ChainstateManager>(chainman_opts, blockman_opts);
        ChainstateManager& chainman = *node.chainman;
        node::ChainstateLoadOptions options;
        options.mempool = Assert(node.mempool.get());