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.
{
    
    const fs::path& datadir = gArgs.GetDataDirNet();
    if (!DirIsWritable(datadir)) {
        return InitError(strprintf(_("Cannot write to data directory '%s'; check permissions."), fs::PathToString(datadir)));
    }
    if (!LockDirectory(datadir, ".lock", probeOnly)) {
        return InitError(strprintf(_("Cannot obtain a lock on data directory %s. %s is probably already running."), fs::PathToString(datadir), PACKAGE_NAME));
    }
    return true;
}
bool AppInitSanityChecks(const kernel::Context& kernel)
{
    
    auto result{kernel::SanityChecks(kernel)};
    if (!result) {
        InitError(util::ErrorString(result));
        return InitError(strprintf(_("Initialization sanity check failed. %s is shutting down."), PACKAGE_NAME));
    }
    
    
    
    return LockDataDirectory(true);
}
bool AppInitLockDataDirectory()