I confess I couldn't quite figure out the branching and tagging strategy on that repo. Very weird stuff. That script seems to have been added by Sebastian Andrzej Siewior just ahead of the 5.6.0 release. It's definitely present in the Debian git tree, and probably in many other distros since others seem to be affected.
The commit where the script was added to Debian is tagged `upstream/v5.6.0` despite the script itself not being present on that tag upstream: https://github.com/tukaani-project/xz/tree/v5.6.0/m4
> I'm also suggesting that there could be more than one exploit present. All of their commits should be rolled back, none of it can be trusted.
I agree.