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_mempool ? &m_mempool->cs : nullptr;
    }
    /**
     * Make mempool consistent after a reorg, by re-adding or recursively erasing
     * disconnected block transactions from the mempool, and also removing any
     * other transactions from the mempool that are no longer valid given the new
     * tip/height.
     *
     * Note: we assume that disconnectpool only contains transactions that are NOT
     * confirmed in the current chain nor already in the mempool (otherwise,
     * in-mempool descendants of such transactions would be removed).
     *
     * Passing fAddToMempool=false will skip trying to add the transactions back,
     * and instead just erase from the mempool as needed.
     */
    void MaybeUpdateMempoolForReorg(
        DisconnectedBlockTransactions& disconnectpool,
        bool fAddToMempool) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_mempool->cs);
    
    void UpdateTip(const CBlockIndex* pindexNew)
        EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
    friend ChainstateManager;
};
/**
 * Provides an interface for creating and interacting with one or two
 * chainstates: an IBD chainstate generated by downloading blocks, and
 * an optional snapshot chainstate loaded from a UTXO snapshot. Managed