Email requests to mods at hn@ycombinator.com.
As HN generally standardises on "X has died" for obituaries, it's possible to search most obituary notices through Algolia. Collating by popularity / votes is a pretty good proxy for significance:
<https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...>
At this writing, Stephen Hawking, Bram Moolenaar, Keven Mitnick, Paul Allen, Queen Elizabeth II, Gordon Moore, John Conway, Dennis Ritchie, and Joe Armstrong are the top ten matches.
Note that this misses a few entries, e.g., Steve Jobs ("has passed away": <https://news.ycombinator.com/item?id=3078128>) and Mike Magee ("Mike Mageek is dead": <https://news.ycombinator.com/item?id=41285851>).
I'd emailed dang regarding the last, noting that not only does the entry not follow the HN "has died" convention, but it misspels (as an in-joke) Magee's name. Dang defended that as in the spirit of the site (and arguably Magee), but it does make searching for the obituary more challenging.
Also worth noting that Stephen Hawking's obit remains the top HN story of all time by votes, for going on eight years.