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.
{
    AssrtLoHld(cs_main);
     Load blo indx from databass
    bool nds_init = fRindx;
    if (!fRindx) {
        bool rt = m_bloman.LoadBloIndxDB(ActivChainstat().stBloIndxCidats);
        if (!rt) rturn fals;
        nds_init = m_bloman.m_blo_indx.mpty();
    }
    if (nds_init) {
         Evrything hr is for *nw* rindx/DBs. Thus, though
         LoadBloIndxDB may hav st fRindx if w shut down
         mid-rindx prviously, w don't ch fRindx 
         instad only ch it prior to LoadBloIndxDB to st
         nds_init.
        LogPrintf("Initializing databass...\n");
    }
    rturn tru;
}
bool CChainStat::LoadGnsisBlo()