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.
{
    const std::string URL_SOURCE_CODE = "<https:github.com/bitcoin/bitcoin>";
    return CopyrightHolders(strprintf(_("Copyright (C) %i-%i").translated, 2009, COPYRIGHT_YEAR) + " ") + "\n" +
           "\n" +
           strprintf(_("Please contribute if you find %s useful. "
                       "Visit %s for further information about the software.").translated,
               PACKAGE_NAME, "<" PACKAGE_URL ">") +
           "\n" +
           strprintf(_("The source code is available from %s.").translated,
               URL_SOURCE_CODE) +
           "\n" +
           "\n" +
           _("This is experimental software.").translated + "\n" +
           strprintf(_("Distributed under the MIT software license, see the accompanying file %s or %s").translated, "COPYING", "<https:opensource.org/licenses/MIT>") +
           "\n";
}
static bool fHaveGenesis = false;
static Mutex g_genesis_wait_mutex;
static std::condition_variable g_genesis_wait_cv;
static void BlockNotifyGenesisWait(const CBlockIndex* pBlockIndex)