The txindex was moved to its own database in #13033. As part of that change, data migration functionality was added, allowing the txindex to be maintained across upgrade without having to rebuild the entire index from scratch:

https://github.com/bitcoin/bitcoin/blob/5cf28d5203a6203355283ea59f071af6a468d239/src/index/txindex.cpp#L75-L90

13033 was included in v0.17, released in October 2018 and EOL in August 2020. Anyone who hasn't upgraded at this point is running unsupported code, and shouldn't have expectations around upgrading their node without having to rebuild the index. We should simplify the code by removing the `MigrateData()` functionality.
argsman.AddArg("-whitelistrelay", strprintf("Add 'relay' permission to whitelisted inbound peers with default permissions. This will accept relayed transactions even when not relaying transactions (default: %d)", DEFAULT_WHITELISTRELAY), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
    argsman.AddArg("-blockmaxweight=<n>", strprintf("Set maximum BIP141 block weight (default: %d)", DEFAULT_BLOCK_MAX_WEIGHT), ArgsManager::ALLOW_ANY, OptionsCategory::BLOCK_CREATION);
    argsman.AddArg("-blockmintxfee=<amt>", strprintf("Set lowest fee rate (in %s/kvB) for transactions to be included in block creation. (default: %s)", CURRENCY_UNIT, FormatMoney(DEFAULT_BLOCK_MIN_TX_FEE)), ArgsManager::ALLOW_ANY, OptionsCategory::BLOCK_CREATION);
    argsman.AddArg("-blockversion=<n>", "Override block version to test forking scenarios", ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::BLOCK_CREATION);
    argsman.AddArg("-rest", strprintf("Accept public REST requests (default: %u)", DEFAULT_REST_ENABLE), ArgsManager::ALLOW_ANY, OptionsCategory::RPC);
    argsman.AddArg("-rpcallowip=<ip>", "Allow JSON-RPC connections from specified source. Valid for <ip> are a single IP (e.g. 1.2.3.4), a network/netmask (e.g. 1.2.3.4/255.255.255.0) or a network/CIDR (e.g. 1.2.3.4/24). This option can be specified multiple times", ArgsManager::ALLOW_ANY, OptionsCategory::RPC);
    argsman.AddArg("-rpcauth=<userpw>", "Username and HMAC-SHA-256 hashed password for JSON-RPC connections. The field <userpw> comes in the format: <USERNAME>:<SALT>$<HASH>. A canonical python script is included in share/rpcauth. The client then connects normally using the rpcuser=<USERNAME>/rpcpassword=<PASSWORD> pair of arguments. This option can be specified multiple times", ArgsManager::ALLOW_ANY | ArgsManager::SENSITIVE, OptionsCategory::RPC);
    argsman.AddArg("-rpcbind=<addr>[:port]", "Bind to given address to listen for JSON-RPC connections. Do not expose the RPC server to untrusted networks such as the public internet! This option is ignored unless -rpcallowip is also passed. Port is optional and overrides -rpcport. Use [host]:port notation for IPv6. This option can be specified multiple times (default: 127.0.0.1 and ::1 i.e., localhost)", ArgsManager::ALLOW_ANY | ArgsManager::NETWORK_ONLY | ArgsManager::SENSITIVE, OptionsCategory::RPC);
    argsman.AddArg("-rpccookiefile=<loc>", "Location of the auth cookie. Relative paths will be prefixed by a net-specific datadir location. (default: data dir)", ArgsManager::ALLOW_ANY, OptionsCategory::RPC);
    argsman.AddArg("-rpcpassword=<pw>", "Password for JSON-RPC connections", ArgsManager::ALLOW_ANY | ArgsManager::SENSITIVE, OptionsCategory::RPC);
    argsman.AddArg("-rpcport=<port>", strprintf("Listen for JSON-RPC connections on <port> (default: %u, testnet: %u, signet: %u, regtest: %u)", defaultBaseParams->RPCPort(), testnetBaseParams->RPCPort(), signetBaseParams->RPCPort(), regtestBaseParams->RPCPort()), ArgsManager::ALLOW_ANY | ArgsManager::NETWORK_ONLY, OptionsCategory::RPC);
    argsman.AddArg("-rpcserialversion", strprintf("Sets the serialization of raw transaction or block hex returned in non-verbose mode, non-segwit(0) or segwit(1) (default: %d)", DEFAULT_RPC_SERIALIZE_VERSION), ArgsManager::ALLOW_ANY, OptionsCategory::RPC);
    argsman.AddArg("-rpcservertimeout=<n>", strprintf("Timeout during HTTP requests (default: %d)", DEFAULT_HTTP_SERVER_TIMEOUT), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::RPC);
    argsman.AddArg("-rpcthreads=<n>", strprintf("Set the number of threads to service RPC calls (default: %d)", DEFAULT_HTTP_THREADS), ArgsManager::ALLOW_ANY, OptionsCategory::RPC);
    argsman.AddArg("-rpcuser=<user>", "Username for JSON-RPC connections", ArgsManager::ALLOW_ANY | ArgsManager::SENSITIVE, OptionsCategory::RPC);
    argsman.AddArg("-rpcwhitelist=<whitelist>", "Set a whitelist to filter incoming RPC calls for a specific user. The field <whitelist> comes in the format: <USERNAME>:<rpc 1>,<rpc 2>,...,<rpc n>. If multiple whitelists are set for a given user, they are set-intersected. See -rpcwhitelistdefault documentation for information on default whitelist behavior.", ArgsManager::ALLOW_ANY, OptionsCategory::RPC);
    argsman.AddArg("-rpcwhitelistdefault", "Sets default behavior for rpc whitelisting. Unless rpcwhitelistdefault is set to 0, if any -rpcwhitelist is set, the rpc server acts as if all rpc users are subject to empty-unless-otherwise-specified whitelists. If rpcwhitelistdefault is set to 1 and no -rpcwhitelist is set, rpc server acts as if all rpc users are subject to empty whitelists.", ArgsManager::ALLOW_BOOL, OptionsCategory::RPC);
    argsman.AddArg("-rpcworkqueue=<n>", strprintf("Set the depth of the work queue to service RPC calls (default: %d)", DEFAULT_HTTP_WORKQUEUE), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::RPC);
    argsman.AddArg("-server", "Accept command line and JSON-RPC commands", ArgsManager::ALLOW_ANY, OptionsCategory::RPC);
#if HAVE_DECL_FORK
    argsman.AddArg("-daemon", strprintf("Run in the background as a daemon and accept commands (default: %d)", DEFAULT_DAEMON), ArgsManager::ALLOW_BOOL, OptionsCategory::OPTIONS);
    argsman.AddArg("-daemonwait", strprintf("Wait for initialization to be finished before exiting. This implies -daemon (default: %d)", DEFAULT_DAEMONWAIT), ArgsManager::ALLOW_BOOL, OptionsCategory::OPTIONS);
#else
    hidden_args.emplace_back("-daemon");
    hidden_args.emplace_back("-daemonwait");
#endif
    
    argsman.AddHiddenArgs(hidden_args);
}
std::string LicenseInfo()