So far, Master Chief has never fully shown his face in the Halo series, others than minor teasers (the closest we get is the Legendary ending in Halo 4) and descriptions in the books. But the upcoming Halo series on Paramount+ will do it differently.
We already knew it would be a separate universe, and in an interview over at IGN, Kiki Wolfkill, Studio Head of Transmedia at 343 Industries, explains that we will get to see Master Chief's face:
"I think we set out to tell a character story and a personal story. And once we really got into what that story was, it became clear that you really needed to see the person in the armor and under the helmet.
You will see his face. For some people, it's been a moment 20 years in the making, and for other people it is something that feels very hard to imagine. We absolutely respect both sides of that fence, those who really want to see Chief's face and those who really don't. But for the nature of this story, it felt really important to connect with the Master Chief in a different way, and that meant showing the face."
This is always a dilemma in movies and shows with helmet wearing heroes like Iron Man and The Mandalorian, and ends up with a lot of unmasked scenes to deliver a more personal story as a face does express a whole lot more than just a helmet.
It should be noted that Master Chief's face isn't a secret for the story and he does take off his helmet in the games as well (even in the ending of Halo: Combat Evolved), but it's never shown to us gamers, and will likely stay this way in the games