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.
#include <txdb.h>
#include <node/ui_interface.h>
#include <pow.h>
#include <random.h>
#include <shutdown.h>
#include <uint256.h>
#include <util/system.h>
#include <util/translation.h>
#include <util/vector.h>
#include <stdint.h>
static constexpr uint8_t DB_COIN{'C'};
static constexpr uint8_t DB_COINS{'c'};
static constexpr uint8_t DB_BLOCK_FILES{'f'};
static constexpr uint8_t DB_BLOCK_INDEX{'b'};
static constexpr uint8_t DB_BEST_BLOCK{'B'};
static constexpr uint8_t DB_HEAD_BLOCKS{'H'};
static constexpr uint8_t DB_FLAG{'F'};
static constexpr uint8_t DB_REINDEX_FLAG{'R'};
static constexpr uint8_t DB_LAST_BLOCK{'l'};
namespace {
struct CoinEntry {
    COutPoint* outpoint;
    uint8_t key;
    explicit CoinEntry(const COutPoint* ptr) : outpoint(const_cast<COutPoint*>(ptr)), key(DB_COIN)  {}