I'm honestly torn on whether this is good news or bad news. On the one hand TIm Longo was creative director on Halo 5 and yikes I can't say I'll miss him, but at the same time creative directors leaving more than a year before launch and after years of development is no a good sign as far as project health goes. I worry about about dev crunch because there is no game that is worth the strain it puts on its developers.
At the very least it will be interesting to see how it all shakes out in the end.
Cyberpunk 2077's creative director left CDPR in January, and CDPR went on to actually give a release date for the game five months later, nor is there any concern about the game's development. It doesn't necessarily mean anything.
Could it mean something? Sure. But it's as likely as any of about a dozen other, benign explanations that have nothing to do with the game. If you see a few high-level managers on the project leave all within a relatively short timeframe, THEN I'll start to worry. If Chris Lee left, I'd worry.
As for crunch, with a launch game that has a largely fixed date, there's one of two possibilities: 1) Devs are crunching to make that date, or 2) They were given a large cushion of time to account for most things that might come up.
Given the extended dev cycle of this game (it'll have been five years since Halo 5 by holiday 2020), the comments from Phil Spencer about giving his teams more time to get things right, and the fact that they evidently have the time and resources to work on the MCC PC port concurrently, I'd guess they are just fine. I believe they also have external studios working on various components of the game, which takes pressure off.
Don't forget they also revamped the game's engine specifically to make it easier to work with.