Quoting: Eike

Quoting: LoudTechieThis is, because the reason behind the whitelist is complicated and thus unsuited for marketing.
At the time of establishment this whitelist contained only CPU's without known side channel attacks.
They really, really wanted to get rid of old side channel attacks for some reason.

After what I gathered from the side channel attack history (which is not too much), I expect every current processor to have some side channel weakness too, though.

I'll be more clear and accurate.
I oversimplified some things.
The moment that whitelist was released.
For all of the processors that weren't on it, there was at that very moment a paper that explained how to extract or change the things even the OS and BIOS don't have access to or it was some obscure processor Win11 was truly incompatible with(non amd64 or ARM), with physical access.
Most of the time these were side channel attacks, because those are conceptional the simplest.

On your expectation. Oh, totally. Many of the processors on the list have been already broken.

Now on why Microsoft might still want to still rid themselves of old attacks.
In the processor secrets: things like somewhat functional DRM, anti-cheat, secure boot and other attempts at limiting software freedom tend to be based on these secrets(Also bitlocker keys, but those were just an excuse).
These things are in basically all their current implementation all backed by security through obscurity. That's also generally vulnerable, but they don't care, since it still slows attacks and is super profitable.
This could be an attempt to placate their government and media backers.