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.
{
    friend CChainState;
private:
    
    void FindFilesToPruneManual(std::set<int>& setFilesToPrune, int nManualPruneHeight, int chain_tip_height);
    /**
     * Prune block and undo files (blk???.dat and undo???.dat) so that the disk space used is less than a user-defined target.
     * The user sets the target (in MB) on the command line or in config file.  This will be run on startup and whenever new
     * space is allocated in a block or undo file, staying below the target. Changing back to unpruned requires a reindex
     * (which in this case means the blockchain must be re-downloaded.)
     *
     * Pruning functions are called from FlushStateToDisk when the global fCheckForPruning flag has been set.
     * Block and undo files are deleted in lock-step (when blk00003.dat is deleted, so is rev00003.dat.)
     * Pruning cannot take place until the longest chain is at least a certain length (100000 on mainnet, 1000 on testnet, 1000 on regtest).
     * Pruning will never delete a block within a defined distance (currently 288) from the active chain's tip.
     * The block index is updated by unsetting HAVE_DATA and HAVE_UNDO for any blocks that were stored in the deleted files.
     * A db flag records the fact that at least some block files have been pruned.
     *
     * @param[out]   setFilesToPrune   The set of file indices that can be unlinked will be returned
     */
    void FindFilesToPrune(std::set<int>& setFilesToPrune, uint64_t nPruneAfterHeight, int chain_tip_height, int prune_height, bool is_ibd);
public:
    BlockMap m_block_index GUARDED_BY(cs_main);
    /** In order to efficiently track invalidity of headers, we keep the set of
      * blocks which we tried to connect and found to be invalid here (ie which
      * were set to BLOCK_FAILED_VALID since the last restart). We can then
      * walk this set and check if a new header is a descendant of something in
      * this set, preventing us from having to walk m_block_index when we try
      * to connect a bad block and fail.