What happens to Fred, who had an appointment and wasn't at the standup? I suppose in this example he'd notice that he isn't able to work, and ask around, but ...
It's imperfect, but IME a standup is more reliable and usable than any alternative I've seen tried. Emails get lost in the noise (that people get too much email at companies is its own problem). A Slack highlight can be more of an interruption, and non-technical people find it hard to use / won't always be responsive there.
Internal wiki hits the same problem as email (although partly because companies always seem to choose terrible wiki implementations (confluence)). Too much information goes on there, terribly organized, and no-one ends up being able to find anything. Mostly these kind of things are immediate announcements that need to be acted on once and never again, so a semi-permanent store isn't appropriate; also the whole point was that everyone needs to be notified, but people can't watch every wiki change.
Blog I haven't seen tried, but I imagine it would hit the same problems; either you notify everyone and have the same problems as slack, or you don't and have a big risk of someone missing an update.
I figured out a work-around. Before I go to my appointment (or vacation day, etc.), I go to the meeting board and put a big green check mark next to all of my completed tasks.
This has an amusing side effect: For some reason, even if I attend the stand-up after all, nobody debates my completed tasks with the big green check marks. So I now do this before every stand-up.
