In my opinion, detail and detailHTML in the wbcheckconstraints API response are a relic from another time. Now that constraints are configured in statements (in a consistent and easily consumable format, not wikitext templates) and violation messages carry all the information you need to understand the violation, I see no need to include them in the API response – the gadget already ignores them.
It looks like this can reduce the response size by about 38% (P6321), which matters not only for what we transfer over the network, but also for what we store in the cache – so I would like to kick this out before we deploy T179849: Cache all constraint check results per-entity.
The announcement for this breaking change was sent on Monday, 20 November 2017, stating that the change will take effect on Monday, 18 December 2017.