Pretty bizarre this ever was stable in the first place.
Unfortunately for this kind of service you need to actively fiddle with the bytes to prevent people from relying on an implementation detail like this and prevent them from digging you into a too big to fail api stability hole.
That's my thought as well. They could also potentially retroactively generate the source tarballs using the old method for every possible repository/tag on Github, store it, and serve that, and then only generate it on-demand for new tags, but I doubt they'll do that. They might though, given this is what led to the problem in the first place (ie; the on-demand generation vs generating on push+storing).
