Part 1:

Yep. The Deck and SteamOS have Secure Boot.

I never said they did not.

I said:

The Steam Deck does not officially support Windows Secure boot.

Because…

Basically, Secure Boot means that … no other OS is allowed to boot.

That’s what ‘Secure’ means, to Windows/MSFT.

Not sure if you struggle with reading comprehension in English, but when you read all of this, together, it is obvious that I am saying that the Windows specific implenentation of Secure Boot is exclusionary, only works with Windows.

This is true, by default, unless you do a bunch of other extra work, which is easy to fuck up and likely to fail at some future point, because the way Windows ‘does’ Secure Boot is very different from how basically every other OS does, and will constantly change in subtle and esoteric ways that often result in a user being unable to access any other OS than Windows.

Windows Secure Boot is thus functionally a distinct thing, even if Windows/MSFT act otherwise and insist on confusing and obfuscatory terminology… which they have a long track record of doing with basically all of their software and related nomenclature, for decades.

Part 2:

Yep, which is why I described that in layman’s terms by saying:

maybe unless you have literally physically distinct harddrives/ssds/microsd/usb drives that each OS lives on?

And then do extra steps to tell your now Windows managed BIOS/UEFI that your linux dual boot OS is also ‘safe’ for Windows to allow your sysyem to boot?

Yep, you can do some extra bullshit, and it might work for a while, untill a new Windows update of some kind rewrites your UEFI config, requires some new arcane dependency setting or config of some kind, which then will lock out your non Windows OS.

Yep, other Mobos often come with everything preconfigured for Windows and their specific implenentation of Secure Boot.

The Steam Deck doesn’t, and that is what we are talking about.

Also, its entirely possible and even common for dual boot and linux users to either intentionally or unintentionally wipe out those Windows EFI files, alter the crypotgraphic signing process in some other way, and then you run into this same problem on other Mobos.