It's not canon

[Sixteen60]

It's a co-op side story so it's not even meant to be canon. And also that apocalypse machine would conflict with Doom series since the modern Doom entries are in the same universe as the MachineGames Wolfenstein series. So basically, Wolfenstein 3 is only gonna be a sequel to The New Colossus and the apocalypse machine will just be ignored so Doom can occur.

[talloyer]

No, Wolfenstein Youngblood IS Canon and Wolfenstein 3's ending is already set on stone.
UNLESS Machinegames manages to bring with a literal retcon with BS plot devices of "time travel" or "multiverse" and treats Wolfenstein: Youngblood as a literal alternate universe of Wolfenstein (as BJ already indicated in this very game) or just a possible future of Wolfenstein 3.

[Wheaton Adams]

It doesn't matter ultimately if it's canon or not. The game is almost a spin-off even if it's part of the same continuity. But ultimately, what matters is: is the game *fun to play?*

Story recently seems to have taken too much of a center stage in games now, and it was particularly visible with New Colossus. The story focus being tyrannical can be a thing, it happened with the Metal Gear franchise.
Youngblood is honestly fun to play and as far as gameplay is concerned, it might be my favorite, bordering on Old Blood if I want different experiences. I will not be afraid to admit New Colossus is actually my least favorite of them all as well. BUT.

It shouldn't matter that much if it's canon or not-canon. It does not make the older titles any less relevant, it does not make them age faster in our minds, and it certainly does not forbid buying them or redownloading them online later on.

I'm honestly a little bit tired of folks who claim "X ruined the franchise because of Y". Can you or friends still buy the previous games? can you still play them? Long as you don't forget them, and can return to them, then they'll never die.
You want a good example of how to kill a franchise? GTA Definitive Edition, where cutting access to the older classics in order to enforce the singular use of a generally worse version for profit,shows how you can kill a franchise. THIS is what it takes in magnitude to ruin a story and franchise.

If Zenimax decided to patch Wolf Old Blood, New Order and New Colossus to make explicit and unnecessary retcons that reference Youngblood that hasn't happened yet in-universe for litterally decades, then yeah, there'd be room to complain too. Metal Gear Solid 4, Peace Walker and 5 did this, and it was a little bit unnecessary.