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 m_coins_views && m_coins_views->m_cacheview;
    }
    
    
    CChain m_chain;
    /**
     * The blockhash which is the base of the snapshot this chainstate was created from.
     *
     * std::nullopt if this chainstate was not created from a snapshot.
     */
    const std::optional<uint256> m_from_snapshot_blockhash;
    /**
     * The set of all CBlockIndex entries with BLOCK_VALID_TRANSACTIONS (for itself and all ancestors) and
     * as good as our current tip or better. Entries may be failed, though, and pruning nodes may be
     * missing the data for the block.
     */
    std::set<CBlockIndex*, CBlockIndexWorkComparator> setBlockIndexCandidates;
    
    CCoinsViewCache& CoinsTip() EXCLUSIVE_LOCKS_REQUIRED(cs_main)
    {
        assert(m_coins_views->m_cacheview);
        return *m_coins_views->m_cacheview.get();
    }
    
    CCoinsViewDB& CoinsDB() EXCLUSIVE_LOCKS_REQUIRED(cs_main)