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.
LOCK(cs_main);
                for (CChainState* chainstate : chainman.GetAll()) {
                    if (!is_coinsview_empty(chainstate)) {
                        uiInterface.InitMessage(_("Verifying blocks…").translated);
                        if (fHavePruned && args.GetArg("-checkblocks", DEFAULT_CHECKBLOCKS) > MIN_BLOCKS_TO_KEEP) {
                            LogPrintf("Prune: pruned datadir may not have more than %d blocks; only checking available blocks\n",
                                MIN_BLOCKS_TO_KEEP);
                        }
                        const CBlockIndex* tip = chainstate->m_chain.Tip();
                        RPCNotifyBlockChange(tip);
                        if (tip && tip->nTime > GetAdjustedTime() + 2 * 60 * 60) {
                            strLoadError = _("The block database contains a block which appears to be from the future. "
                                    "This may be due to your computer's date and time being set incorrectly. "
                                    "Only rebuild the block database if you are sure that your computer's date and time are correct");
                            failed_verification = true;
                            break;
                        }
                        if (!CVerifyDB().VerifyDB(
                                *chainstate, chainparams, chainstate->CoinsDB(),
                                args.GetArg("-checklevel", DEFAULT_CHECKLEVEL),
                                args.GetArg("-checkblocks", DEFAULT_CHECKBLOCKS))) {
                            strLoadError = _("Corrupted block database detected");
                            failed_verification = true;
                            break;
                        }
                    }
                }
            } catch (const std::exception& e) {
                LogPrintf("%s\n", e.what());
                strLoadError = _("Error opening block database");