ACP > System > Check for updates
I believe this page, as well as docs/Install.html and any appropriate pages on phpbb.com should be updated to instruct users to disable the board before doing updates or upgrades, and to reenable it afterward.
It is possible that while following the current procedures, users will experience errors as the /cache and /vendor folders are deleted or new software is being uploaded. When the /install folder is uploaded, the board effectively becomes disabled but that can take some time.
But also, this allows a consistent database backup to occur. On large boards the phpbb_forums table may be backed up long before the phpbb_posts table is, leading to a potentially inconsistent database if the database has to be recovered.