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.
{
    rpc_notify_block_change_connection.disconnect();
    RPCNotifyBlockChange(nullptr);
    g_best_block_cv.notify_all();
    LogPrint(BCLog::RPC, "RPC stopped.\n");
}
void SetupServerArgs(ArgsManager& argsman)
{
    SetupHelpOptions(argsman);
    argsman.AddArg("-help-debug", "Print help message with debugging options and exit", ArgsManager::ALLOW_ANY, OptionsCategory::DEBUG_TEST); 
    init::AddLoggingArgs(argsman);
    const auto defaultBaseParams = CreateBaseChainParams(CBaseChainParams::MAIN);
    const auto testnetBaseParams = CreateBaseChainParams(CBaseChainParams::TESTNET);
    const auto signetBaseParams = CreateBaseChainParams(CBaseChainParams::SIGNET);
    const auto regtestBaseParams = CreateBaseChainParams(CBaseChainParams::REGTEST);
    const auto defaultChainParams = CreateChainParams(argsman, CBaseChainParams::MAIN);
    const auto testnetChainParams = CreateChainParams(argsman, CBaseChainParams::TESTNET);
    const auto signetChainParams = CreateChainParams(argsman, CBaseChainParams::SIGNET);
    const auto regtestChainParams = CreateChainParams(argsman, CBaseChainParams::REGTEST);
    
    std::vector<std::string> hidden_args = {
        "-dbcrashratio", "-forcecompactdb",
        
        "-choosedatadir", "-lang=<lang>", "-min", "-resetguisettings", "-splash", "-uiplatform"};
    argsman.AddArg("-version", "Print version and exit", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
#if HAVE_SYSTEM
    argsman.AddArg("-alertnotify=<cmd>", "Execute command when a relevant alert is received or we see a really long fork (%s in cmd is replaced by message)", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
#endif