I'm a bit conflicted about the best way to handle these kind of things, but I thought it might be good to collect the existing MO "memorial" questions in a thread here on meta, in case some decision is reached (e.g., to give them all a certain tag). Here are some I've found:

https://mathoverflow.net/questions/81821/in-memoriam-torsten-ekedahl [This one concerned an active MO user and so might be considered somewhat different.]

https://mathoverflow.net/questions/187106/grothendieck-sad-news

https://mathoverflow.net/questions/199212/maryam-mirzakhanis-works [Whoops, this one was not actually asked after her death, only edited to reflect that.]

https://mathoverflow.net/questions/282420/vladimir-voevodskys-works

https://mathoverflow.net/questions/319893/jean-bourgains-relatively-lesser-known-significant-contributions

https://mathoverflow.net/questions/357197/conways-lesser-known-results

https://mathoverflow.net/questions/365170/ron-l-graham-s-lesser-known-significant-contributions

https://mathoverflow.net/questions/371275/lesser-known-results-by-vaughan-jones

If you know of more examples, please add them here.