Just in case anyone else is as clueless as me, if you can't find a link to code updates in the tracker: Go to https://github.com/moodle/moodle and enter the issue number in the Search box. The link to the code change is under Commits.
It seems so obvious now, but not so much when I was panicking late last night... Thanks again for your prompt assistance Tim.
The issue was identified because forum posts were not being emailed. It has temporarily been resolved by disabling \core\task\legacy_plugin_cron_task - but what effect will this have on Quizzes or any other mods that may be using legacy cron?
To be honest, I don't remember exactly what the situation was in Moodle 3.5.
There was a big push to get all the legacy cron converted to use the new scheduled task system (MDL-61164). Some of that was done for 3.5, but some of it was not finished until 3.7 (See the linked issues.)