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.
return false;
    }
     Either install a handler to notify us when genesis activates, or set fHaveGenesis directly.
     No locking, as this happens before any background thread is started.
    boost::signals2::connection block_notify_genesis_wait_connection;
    if (chainman.ActiveChain().Tip() == nullptr) {
        block_notify_genesis_wait_connection = uiInterface.NotifyBlockTip_connect(std::bind(BlockNotifyGenesisWait, std::placeholders::_2));
    } else {
        fHaveGenesis = true;
    }
#if HAVE_SYSTEM
    const std::string block_notify = args.GetArg("-blocknotify", "");
    if (!block_notify.empty()) {
        uiInterface.NotifyBlockTip_connect([block_notify](SynchronizationState sync_state, const CBlockIndex* pBlockIndex) {
            if (sync_state != SynchronizationState::POST_INIT || !pBlockIndex) return;
            std::string command = block_notify;
            boost::replace_all(command, "%s", pBlockIndex->GetBlockHash().GetHex());
            std::thread t(runCommand, command);
            t.detach(); 
        });
    }
#endif
    std::vector<fs::path> vImportFiles;
    for (const std::string& strFile : args.GetArgs("-loadblock")) {
        vImportFiles.push_back(strFile);
    }
    chainman.m_load_block = std::thread(&util::TraceThread, "loadblk", [=, &chainman, &args] {
        ThreadImport(chainman, vImportFiles, args);
    });
     Wait for genesis block to be processed