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.
hidden_args.emplace_back("-zmqpubhashblockhwm=<n>");
    hidden_args.emplace_back("-zmqpubhashtxhwm=<n>");
    hidden_args.emplace_back("-zmqpubrawblockhwm=<n>");
    hidden_args.emplace_back("-zmqpubrawtxhwm=<n>");
    hidden_args.emplace_back("-zmqpubsequencehwm=<n>");
#endif
    argsman.AddArg("-checkblocks=<n>", strprintf("How many blocks to check at startup (default: %u, 0 = all)", DEFAULT_CHECKBLOCKS), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-checklevel=<n>", strprintf("How thorough the block verification of -checkblocks is: %s (0-4, default: %u)", Join(CHECKLEVEL_DOC, ", "), DEFAULT_CHECKLEVEL), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-checkblockindex", strprintf("Do a consistency check for the block tree, chainstate, and other validation data structures occasionally. (default: %u, regtest: %u)", defaultChainParams->DefaultConsistencyChecks(), regtestChainParams->DefaultConsistencyChecks()), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-checkaddrman=<n>", strprintf("Run addrman consistency checks every <n> operations. Use 0 to disable. (default: %u)", DEFAULT_ADDRMAN_CONSISTENCY_CHECKS), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-checkmempool=<n>", strprintf("Run mempool consistency checks every <n> transactions. Use 0 to disable. (default: %u, regtest: %u)", defaultChainParams->DefaultConsistencyChecks(), regtestChainParams->DefaultConsistencyChecks()), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-checkpoints", strprintf("Enable rejection of any forks from the known historical chain until block %s (default: %u)", defaultChainParams->Checkpoints().GetHeight(), DEFAULT_CHECKPOINTS_ENABLED), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-deprecatedrpc=<method>", "Allows deprecated RPC method(s) to be used", ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-stopafterblockimport", strprintf("Stop running after importing blocks from disk (default: %u)", DEFAULT_STOPAFTERBLOCKIMPORT), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-stopatheight", strprintf("Stop running after reaching the given height in the main chain (default: %u)", DEFAULT_STOPATHEIGHT), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-limitancestorcount=<n>", strprintf("Do not accept transactions if number of in-mempool ancestors is <n> or more (default: %u)", DEFAULT_ANCESTOR_LIMIT), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-limitancestorsize=<n>", strprintf("Do not accept transactions whose size with all in-mempool ancestors exceeds <n> kilobytes (default: %u)", DEFAULT_ANCESTOR_SIZE_LIMIT_KVB), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-limitdescendantcount=<n>", strprintf("Do not accept transactions if any ancestor would have <n> or more in-mempool descendants (default: %u)", DEFAULT_DESCENDANT_LIMIT), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-limitdescendantsize=<n>", strprintf("Do not accept transactions if any ancestor would have more than <n> kilobytes of in-mempool descendants (default: %u).", DEFAULT_DESCENDANT_SIZE_LIMIT_KVB), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-addrmantest", "Allows to test address relay on localhost", ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-capturemessages", "Capture all P2P messages to disk", ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-mocktime=<n>", "Replace actual time with " + UNIX_EPOCH_TIME + " (default: 0)", ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-maxsigcachesize=<n>", strprintf("Limit sum of signature cache and script execution cache sizes to <n> MiB (default: %u)", DEFAULT_MAX_SIG_CACHE_BYTES >> 20), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-maxtipage=<n>",
                   strprintf("Maximum tip age in seconds to consider node in initial block download (default: %u)",
                             Ticks<std::chrono::seconds>(DEFAULT_MAX_TIP_AGE)),
                   ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-printpriority", strprintf("Log transaction fee rate in " + CURRENCY_UNIT + "kvB when mining blocks (default: %u)", DEFAULT_PRINTPRIORITY), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
    argsman.AddArg("-uacomment=<cmt>", "Append comment to the user agent string", ArgsManager::ALLOW_ANY, OptionsCategory::DEBUG_TEST);
    SetupChainParamsBaseOptions(argsman);