Alternatively, they could extract the compression code and maintain it for repo tags created before the git algo update release date.
Isn’t that the only humane course given all that depends on this?
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).
That seems wasteful. Many projects do not actively advertise the GitHub tag downloads, and instead have their own stored and stable tarballs (or other distributions). And I suppose many users of those auto-generated downloads don’t care about their checksums.
The solution is simple: for a monthly payment of $1 per gigabyte the downloads are stable. Otherwise they are not.
Who should pay for this? The developer of the project might not care about the stability, but the maintainers of the various Linux distros might. Should the developer pay from their own pocket to make the Linux folks happy? Should there be some pool of all the Linux distros to collect the fees and pay for these projects?
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.