`RewindBlockIndex` was made to allow nodes upgrading to new consensus rules post-segwit fork to 
download new consensus data (i.e witnesses) without having to go through a whole IBD. It has been pointed than now really few pre-segwit nodes care to upgrade and that this logic may be irrelevant.

See https://github.com/bitcoin/bitcoin/pull/17737#discussion_r358514335 in review  of #17737 for more context pointers.
{
    
    fs::path datadir = GetDataDir();
    if (!DirIsWritable(datadir)) {
        return InitError(strprintf(_("Cannot write to data directory '%s'; check permissions."), datadir.string()));
    }
    if (!LockDirectory(datadir, ".lock", probeOnly)) {
        return InitError(strprintf(_("Cannot obtain a lock on data directory %s. %s is probably already running."), datadir.string(), PACKAGE_NAME));
    }
    return true;
}
bool AppInitSanityChecks()
{
    
    init::SetGlobals();
    if (!init::SanityChecks()) {
        return InitError(strprintf(_("Initialization sanity check failed. %s is shutting down."), PACKAGE_NAME));
    }
    
    
    
    return LockDataDirectory(true);
}
bool AppInitLockDataDirectory()