There are some commits in the presync branch of McLachlan that modify generated code. They can be found via
git log ML_*/ or by regenerating the code and checking what changed. None of these changes will survive the mandatory regeneration of ML’s code before the release (see https://docs.einsteintoolkit.org/et-docs/Release_Process#One_or_two_days_before_the_release).
In order to survive they must all be (somehow) incorporated into the Mathematica sources or possibly use the “MergeFiles” inclusion mechanism present in Kranc (see ML_ADMConstraints.m and its
MergeFiles → “prototype/ML_ADMConstraints” line). The generated C code may require changes to Kranc.