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.
options.reindex = node::fReindex;
        options.reindex_chainstate = fReindexChainState;
        options.prune = chainman.m_blockman.IsPruneMode();
        options.check_blocks = args.GetIntArg("-checkblocks", DEFAULT_CHECKBLOCKS);
        options.check_level = args.GetIntArg("-checklevel", DEFAULT_CHECKLEVEL);
        options.require_full_verification = args.IsArgSet("-checkblocks") || args.IsArgSet("-checklevel");
        options.check_interrupt = ShutdownRequested;
        options.coins_error_cb = [] {
            uiInterface.ThreadSafeMessageBox(
                _("Error reading from database, shutting down."),
                "", CClientUIInterface::MSG_ERROR);
        };
        uiInterface.InitMessage(_("Loading block index…").translated);
        const auto load_block_index_start_time{SteadyClock::now()};
        auto catch_exceptions = [](auto&& f) {
            try {
                return f();
            } catch (const std::exception& e) {
                LogPrintf("%s\n", e.what());
                return std::make_tuple(node::ChainstateLoadStatus::FAILURE, _("Error opening block database"));
            }
        };
        auto [status, error] = catch_exceptions([&]{ return LoadChainstate(chainman, cache_sizes, options); });
        if (status == node::ChainstateLoadStatus::SUCCESS) {
            uiInterface.InitMessage(_("Verifying blocks…").translated);
            if (chainman.m_blockman.m_have_pruned && options.check_blocks > MIN_BLOCKS_TO_KEEP) {
                LogPrintfCategory(BCLog::PRUNE, "pruned datadir may not have more than %d blocks; only checking available blocks\n",
                                  MIN_BLOCKS_TO_KEEP);
            }
            std::tie(status, error) = catch_exceptions([&]{ return VerifyLoadedChainstate(chainman, options);});