Halo community manager Brian Jarrard made a statement on Reddit today confirming that Halo 6 will not be at E3."[W]e'll have a little something at E3 but it's not related to the next major entry in the franchise,"
Many were expecting an announcement for Halo 6 at E3 this year despite knowing Microsoft confirmed a Halo FPS title will not release in 2017. Fans expected a small teaser with a potential fall 2018 release window but it won't be happening.
It's not incredibly surprising that the reveal won't be happening as Xbox has a lot on their plate already. They'll be talking about Project Scorpio in great detail, revealing gameplay and release dates for a plethora of previously announced games, and they'll probably have a lot of unannounced games to talk about.
It's probably a good sign that 343 is taking their time with Halo 6 and is listening to fan feedback. The community had a lot of complaints regarding Halo 5's story, it was short, didn't deliver on all of the promises, and let a lot of people down. The multiplayer side of things seemed to be great from the get-go and has been supported continuously but Halo's campaign has always been near and dear to people's hearts.
As for the "little something", it's possible that they are teasing the heavily rumored Halo 3 remaster which may come to PC, it could be a new piece of DLC for Halo War 2, or the Halo TV series produced by Steven Spielberg could see a revival! This is all pure speculation but it's something to consider.
Xbox's E3 show is set for Sunday, June 11th.