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.
/**
* Validation result for a single transaction mempool acceptance.
*/
struct MempoolAcceptResult {
    
    enum class ResultType {
        VALID, 
        INVALID, 
    };
    const ResultType m_result_type;
    const TxValidationState m_state;
    
    
    const std::optional<std::list<CTransactionRef>> m_replaced_transactions;
    
    const std::optional<CAmount> m_base_fees;
    static MempoolAcceptResult Failure(TxValidationState state) {
        return MempoolAcceptResult(state);
    }
    static MempoolAcceptResult Success(std::list<CTransactionRef>&& replaced_txns, CAmount fees) {
        return MempoolAcceptResult(std::move(replaced_txns), fees);
    }
private:
    
    explicit MempoolAcceptResult(TxValidationState state)
        : m_result_type(ResultType::INVALID), m_state(state) {
            Assume(!state.IsValid()); 
        }
    
    explicit MempoolAcceptResult(std::list<CTransactionRef>&& replaced_txns, CAmount fees)