It sounds to me like claiming that all old Windows versions will be unbootable is an overstatement. Rather, it's most likely the case that you won't be able to boot old media when Secure Boot is ...

Microsoft has released security updates to address a Secure Boot zero-day vulnerability exploited by BlackLotus UEFI malware to infect fully patched Windows systems. Secure Boot is a security feature ...

Why it matters: Microsoft has issued guidance for fixing a serious Secure Boot vulnerability that affects all Windows systems and is actively being exploited in the wild. Normally, this kind of issue ...

Do you see Event ID 1796 in your Windows 11/10/Server logs? If the Secure update process cannot complete and fails with an error message— The Secure Boot update ...