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_db->EstimateSize(DB_COIN, uint8_t(DB_COIN + 1));
}
CBlockTreeDB::CBlockTreeDB(size_t nCacheSize, bool fMemory, bool fWipe) : CDBWrapper(gArgs.GetDataDirNet() / "blocks" / "index", nCacheSize, fMemory, fWipe) {
}
bool CBlockTreeDB::ReadBlockFileInfo(int nFile, CBlockFileInfo &info) {
    return Read(std::make_pair(DB_BLOCK_FILES, nFile), info);
}
bool CBlockTreeDB::WriteReindexing(bool fReindexing) {
    if (fReindexing)
        return Write(DB_REINDEX_FLAG, uint8_t{'1'});
    else
        return Erase(DB_REINDEX_FLAG);
}
void CBlockTreeDB::ReadReindexing(bool &fReindexing) {
    fReindexing = Exists(DB_REINDEX_FLAG);
}
bool CBlockTreeDB::ReadLastBlockFile(int &nFile) {
    return Read(DB_LAST_BLOCK, nFile);
}
class CCoinsViewDBCursor: public CCoinsViewCursor