Australia has long had a contentious relationship with video games and a number of titles are currently being refused classification. Among them is an upcoming Rockstar product codenamed Bonaire, which may or may not be related to Red Dead Redemption 2.
There's no information of any actual significance in terms of what it entails. All we know is that Bonaire: (1) is a game, or content for a game, produced by Rockstar; (2) is internally referred to as Bonaire; and (3) that it apparently contains the kind of content that would get it refused classification in Australia.
This comes right after the recent banning of DayZ and the re-banning of We Happy Few after a brief period of free reign. Australia in particular jumps at any depiction of drug usage in games. Present in limited measures in Red Dead Redemption 2, the depiction of drugs in Rockstar's wild west epic is mild enough to let it fly.
However, if the prevailing theory about what Bonaire entails is correct, it could explain why it got refused classification.
Bonaire might refer to the Caribbean, as there is an island located in the famous island cluster named Bonaire. Arthur Morgan and co visit the Caribbean during the course of Red Dead Redemption 2's storyline.
If Bonaire really is related to Red Dead Redemption 2, it may be the codename for upcoming new content, either for Online or for the single-player portion, significant enough to require its own rating - this suggests a larger single-player expansion, think Undead Nightmare.
Considering the time period the game is set in, combined with the Caribbean location and the general themes of outlaws and crime, you get a pretty obvious setup for a story that might involve illicit drug trade, potentially depicting substance abuse prominently enough to tick off the folks Down Under.
Of course, it could be something utterly different - an entirely new IP, or possibly a sequel to Bully that's been also rumored.
Lacking any official information, we can only speculate as to what Bonaire could be. But if it's being classified, an announcement might be coming soon.