A plugin update rollback, and the failure it caught

6.6 keeps a copy of the previous plugin version during an update and restores it if the update produces a fatal error.

what happened, on a Wednesday:

  a plugin updated, its new version required PHP 8.3,
  and the host was on 8.2. the plugin fataled on
  activation.

  before 6.6: a white screen, and a manual restore
              over SFTP.
  with 6.6:   the update rolled back, the old version
              reactivated, and an admin notice
              explaining it.

total downtime: none.

The rollback covers a fatal error during the update and not a plugin that activates cleanly and misbehaves afterwards, which is the more common failure — so this is a floor rather than a safety net. It also means a plugin update can now silently not happen, and the notice is the only signal, which is worth checking after any bulk update.