> In theory I could use an old version of Hugo, but I have no idea when it broke, so how far do I go back?
Had the same problem. Binary search is the latest trick people use.
For SSG there's not much point in upgrading if everything works, and planned migration beats the churn in this case.
You can just print a Hugo version in a HTML comment to track it in git.
Binary search is a very old trick, going back to 1946 on computers, and probably thousands of years before that, since searching sorted lists goes back to at least ancient Babylon. https://en.wikipedia.org/wiki/Binary_search
There's a "git bisect" command waiting to get in the game