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.
{
protected:
    class DB;
private:
    const std::unique_ptr<DB> m_db;
protected:
    
    bool Init() override;
    bool WriteBlock(const CBlock& block, const CBlockIndex* pindex) override;
    BaseIndex::DB& GetDB() const override;
    const char* GetName() const override { return "txindex"; }
public:
    
    explicit TxIndex(size_t n_cache_size, bool f_memory = false, bool f_wipe = false);
    
    virtual ~TxIndex() override;
    
    
    
    
    
    
    bool FindTx(const uint256& tx_hash, uint256& block_hash, CTransactionRef& tx) const;
};
extern std::unique_ptr<TxIndex> g_txindex;
#endif