We deployed afform ages ago but I only just realised we are still using our old version, not the one that ships with core. That would explain why this patch https://github.com/eileenmcnaughton/deduper/pull/9 broke rather than fixed our site
So we need to 1 - remove afform from sites/default/civicrm/extensions
- ensure deduper & contact layout editor are working. I expect that involves updates to them including ^^