As reported at https://www.mediawiki.org/wiki/User_talk:Krinkle#GUC_Tool_error,_or?
On July 10 I sent a mass message about Abstract Wikipedia to all 150+ affiliates. A manual random check confirmed that messages were delivered.
On July 13, running https://guc.toolforge.org/?by=date&user=MediaWiki_message_delivery, all I could find in the log (for my delivery, not for others), was:
(diff | hist) 16:51, 10 Jul 2020 . . ua.wikimedia.org . . MediaWiki message delivery (talk | contribs) . . Обговорення:Вікімедіа Україна (→Announcing a new wiki project! Welcome, Abstract Wikipedia: нова тема) (current) (diff | hist) 16:51, 10 Jul 2020 . . pl.wikimedia.org . . MediaWiki message delivery (talk | contribs) . . Dyskusja:Strona główna (→Announcing a new wiki project! Welcome, Abstract Wikipedia: nowa sekcja) (current) (diff | hist) 16:51, 10 Jul 2020 . . no.wikimedia.org . . MediaWiki message delivery (talk | contribs) . . Wikimedia:Samfunnet (→Announcing a new wiki project! Welcome, Abstract Wikipedia: ny seksjon) (current) (diff | hist) 16:51, 10 Jul 2020 . . nl.wikimedia.org . . MediaWiki message delivery (talk | contribs) . . Wikimedia:De kroeg (→Announcing a new wiki project! Welcome, Abstract Wikipedia: nieuwe subkop) (current)
On August 3, even the entry for pl.wikimedia is actually missing. Most if not all of those pages were not targeted by other messages or even just edited in the meantime (first example I could think of).
I rely on GUC for MassMessage to
- verify that message has been indeed delivered, possibly once, and
- verify shortly after delivery (1-2 days after) which pages were lated edited (as they would not feature the "current" tag) and manually check them for potential replies to my mass message.