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("-whitebind=<[permissions@]addr>", "Bind to the given address and add permission flags to the peers connecting to it. "
        "Use [host]:port notation for IPv6. Allowed permissions: " + Join(NET_PERMISSIONS_DOC, ", ") + ". "
        "Specify multiple permissions separated by commas (default: download,noban,mempool,relay). Can be specified multiple times.", ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
    argsman.AddArg("-whitelist=<[permissions@]IP address or network>", "Add permission flags to the peers connecting from the given IP address (e.g. 1.2.3.4) or "
        "CIDR-notated network (e.g. 1.2.3.0/24). Uses the same permissions as "
        "-whitebind. Can be specified multiple times." , ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
    g_wallet_init_interface.AddWalletOptions(argsman);
#if ENABLE_ZMQ
    argsman.AddArg("-zmqpubhashblock=<address>", "Enable publish hash block in <address>", ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubhashtx=<address>", "Enable publish hash transaction in <address>", ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubrawblock=<address>", "Enable publish raw block in <address>", ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubrawtx=<address>", "Enable publish raw transaction in <address>", ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubsequence=<address>", "Enable publish hash block and tx sequence in <address>", ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubhashblockhwm=<n>", strprintf("Set publish hash block outbound message high water mark (default: %d)", CZMQAbstractNotifier::DEFAULT_ZMQ_SNDHWM), ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubhashtxhwm=<n>", strprintf("Set publish hash transaction outbound message high water mark (default: %d)", CZMQAbstractNotifier::DEFAULT_ZMQ_SNDHWM), ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubrawblockhwm=<n>", strprintf("Set publish raw block outbound message high water mark (default: %d)", CZMQAbstractNotifier::DEFAULT_ZMQ_SNDHWM), ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubrawtxhwm=<n>", strprintf("Set publish raw transaction outbound message high water mark (default: %d)", CZMQAbstractNotifier::DEFAULT_ZMQ_SNDHWM), ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubsequencehwm=<n>", strprintf("Set publish hash sequence message high water mark (default: %d)", CZMQAbstractNotifier::DEFAULT_ZMQ_SNDHWM), ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
#else
    hidden_args.emplace_back("-zmqpubhashblock=<address>");
    hidden_args.emplace_back("-zmqpubhashtx=<address>");
    hidden_args.emplace_back("-zmqpubrawblock=<address>");
    hidden_args.emplace_back("-zmqpubrawtx=<address>");
    hidden_args.emplace_back("-zmqpubsequence=<n>");
    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