This is the first time in the Halo franchise we have not seen some gameplay demo in less then a year before launch, or even a ViDoc at least.
Some may argue that development may be at risk, and 343i are waiting till the very end to show it because it could damage them in a very bad way if they show it in a early/buggy state.
They could not be wrong, especially after Halo 5 Campaign fiasco.
But personally, as an avid fan since CE E3 demo on a friggin' Mac, I believe this time it's more likely due to the new "Slipstream" engine.
This is also the first time an Halo game took over 5 years of development to ship mainly for this reason.
For the first time in 20 years, the main "Blam!", extremely re-iterated and improved among generation, Halo engine got actually totally replaced from scratch by a new one with monster devices like XSX and current best high-end PC as the new baseline in mind.
This reason alone is a monumental effort for a features-packed game like Halo, and an huge bet to be able to keep the best of previous engine and games while introducing all the new advantages and innovation the new one could offer.
This, and the fact Halo Infinite is 100% tied with the launch and showcase of what XSX can actual deliver have delayed that gameplay demo to the very end to me (last E3 before shipping the new console) , where also many other next-gen games will finally get unveiled.
Microsoft is actually trying to pull off another "OG Xbox + Halo CE" proposal and success.
And, in my opinion, everything is pointing toward the fact they may actually do it again.