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.
using kernel::ValidationCacheSizes;
using node::ApplyArgsManOptions;
using node::BlockManager;
using node::CacheSizes;
using node::CalculateCacheSizes;
using node::DEFAULT_PERSIST_MEMPOOL;
using node::DEFAULT_PRINTPRIORITY;
using node::fReindex;
using node::g_indexes_ready_to_sync;
using node::KernelNotifications;
using node::LoadChainstate;
using node::MempoolPath;
using node::NodeContext;
using node::ShouldPersistMempool;
using node::ThreadImport;
using node::VerifyLoadedChainstate;
static constexpr bool DEFAULT_PROXYRANDOMIZE{true};
static constexpr bool DEFAULT_REST_ENABLE{false};
static constexpr bool DEFAULT_I2P_ACCEPT_INCOMING{true};
#ifdef WIN32
#define MIN_CORE_FILEDESCRIPTORS 0
#else
#define MIN_CORE_FILEDESCRIPTORS 150
#endif
static const char* DEFAULT_ASMAP_FILENAME="ip_asn.map";
**
 * The PID file facilities.
 *
static const char* BITCOIN_PID_FILENAME = "bitcoind.pid";
static fs::path GetPidFile(const ArgsManager& args)