I don't think it's appropriate to make such an announcement EDIT: on the main site. I understand the impulse, since news like this propagates slower than you might think. My recollection is that in a few cases where a member of the MO community died, we added a banner to the top of the site, but posting this as a normal question is simply not what the site is for (this is easily seen from: https://mathoverflow.net/help/on-topic). I think this is particularly clear since Yoccoz didn't have any connection I know of to MO, but in general, I don't think we want to be in the business of sorting out which such events to announce and which not.
I think it's fine to make a post in meta about someone involved in the community passing away (or has a more happy life event; maybe I should have posted when my daughter was born). I think posting about every mathematician who dies is not really what meta is intended for, but I doubt anyone will object to it from time to time.
EDIT: I have to admit, I'm slightly confused about the downvotes. There are a lot of gray areas in what is an acceptable question and what isn't, but announcements are very clearly outside site policy. I understand the impulse, because there aren't a lot of places to make announcements to a broad audience of mathematicians, but there are a lot of announcements that people out in the world want to make, and they could easily drown out the questions (most of which are much less notable than this news).
Maybe I muddied the waters too much by talking about whether it was appropriate to post on Meta. I think if you read the explanation of what Meta is (https://mathoverflow.net/help/whats-meta), then my reading is that announcements are not really part of the intended purpose, but Meta doesn't really have a clear policy on what's kosher and what not, so it's harder to make firm pronouncements.