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;
        }
    }
     ********************************************************* Step 9: load wallet
    for (const auto& client : node.chain_clients) {
        if (!client->load()) {
            return false;
        }
    }
     ***
     if pruning, unset the service bit and perform the initial blockstore prune
     after any wallet rescanning has taken place.
    if (fPruneMode) {
        LogPrintf("Unsetting NODE_NETWORK on prune mode\n");
        nLocalServices = ServiceFlags(nLocalServices & ~NODE_NETWORK);
        if (!fReindex) {
            LOCK(cs_main);
            for (CChainState* chainstate : chainman.GetAll()) {
                uiInterface.InitMessage(_("Pruning blockstore…").translated);
                chainstate->PruneAndFlush();
            }
        }
    }
     ********************************************************* Step 11: import blocks
    if (!CheckDiskSpace(gArgs.GetDataDirNet())) {
        InitError(strprintf(_("Error: Disk space is low for %s"), gArgs.GetDataDirNet()));
        return false;
    }
    if (!CheckDiskSpace(gArgs.GetBlocksDirPath())) {
        InitError(strprintf(_("Error: Disk space is low for %s"), gArgs.GetBlocksDirPath()));