That would have been the sensible thing to do. Unfortunately by that point of the day my brain went for the nuclear option and just did a quick scan of the old child theme to work out what was in there, figured out a workaround for those bits and then nuked the child theme. This test site was an old legacy site so the child theme was about 5 years old and Divi had moved on a lot since then so most of the bits were no longer needed.
That said, I am only half way through rolling the latest WP out through my maintenance clients (I always do these major rollouts manually and test any of the big sites on clone test locations first). Some of their sites are about the same vintage. If I get the same error I will do a more careful comb through the code.