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 InitError(Untranslated("rpcserialversion must be non-negative."));
    if (args.GetIntArg("-rpcserialversion", DEFAULT_RPC_SERIALIZE_VERSION) > 1)
        return InitError(Untranslated("Unknown rpcserialversion requested."));
#if defined(USE_SYSCALL_SANDBOX)
    if (args.IsArgSet("-sandbox") && !args.IsArgNegated("-sandbox")) {
        const std::string sandbox_arg{args.GetArg("-sandbox", "")};
        bool log_syscall_violation_before_terminating{false};
        if (sandbox_arg == "log-and-abort") {
            log_syscall_violation_before_terminating = true;
        } else if (sandbox_arg == "abort") {
            
        } else {
            return InitError(Untranslated("Unknown syscall sandbox mode (-sandbox=<mode>). Available modes are \"log-and-abort\" and \"abort\"."));
        }
        
        const std::vector<std::string> features_using_execve{
            "-alertnotify",
            "-blocknotify",
            "-signer",
            "-startupnotify",
            "-walletnotify",
        };
        for (const std::string& feature_using_execve : features_using_execve) {
            if (!args.GetArg(feature_using_execve, "").empty()) {
                return InitError(Untranslated(strprintf("The experimental syscall sandbox feature (-sandbox=<mode>) is incompatible with %s (which uses execve).", feature_using_execve)));
            }
        }
        if (!SetupSyscallSandbox(log_syscall_violation_before_terminating)) {
            return InitError(Untranslated("Installation of the syscall sandbox failed."));
        }