While these Notifications might be informative and perhaps useful toImpudent response: sounds like your members need to learn what the Mute link in the message footer is for.
In fact I don't have a problem with having a group option to Mute the new system notification hashtags for all members, as each new hashtag is implemented. Retroactively perhaps for the three recent ones (#file, #photo & #wiki). But I wouldn't do that to the well established #cal-xxx hashtags.
I'm not exactly sure what form such a control might take. Given that the list of system notification messages is likely to remain relatively short (eight or ten now, maybe a score later?) I was thinking it could be added as a Default Notification Settings panel in the group's Default Sub Settings tab.
But that doesn't address the question of how to apply it to existing members. That might need to be done as a one-time sweep by Groups.io, at applying this new panel to existing members, with the notification hashtags default set to Mute (Forever).