Please, don't forget to update pages on their entirety when you want to reflect a certain change made to the subject of an article. For instance, if a character dies, do not simply add the character's appearance of death to the infobox in their article; it's mandatory that the addition is supported by the information available in the History section.
Adding to every mutant with an unknown status that their M-Day status is unknown isn't useful. We should stick to what we know, supported with references (some of them indeed stating an unknown M-Day status).
But if you want to work on the M-Day, I'm pretty sure most of the related pages needs either references, restructuration or (referenced) content.
I disagree that it is not useful. I found similar notes very useful when I saw them on the pages for Anti-Matter and Amber, because that was exactly the information I was looking for. (in fact, I copied the text I was adding from the Anti-Matter page.)
I also disagree that I am adding something we don't know. We know that M-Day happened and it was important to all mutants, which is all my addition really says.
But I do understand your general objection to un-referenced information. So I'll stop for now, until I can find a good reference.