Flag the deferred integration concerns explicitly.

Revision history, Special:RecentChanges, watchlist, Special:WhatLinksHere, and XML dumps are all downstream consumers of the article read path that will see opted-in abstract articles inconsistently (or not at all) until those integrations are built. M2 deliberately does not build them — the M2 header lists them as known-deferred — but this bullet is the right place to record that the deferral is visible and tracked, so that a post-pilot triage can pick them up based on what pilot feedback actually shows to be blocking.