Find the Secure Boot option and change it to Disabled. Save the changes and reboot again. We recommend keeping Secure Boot enabled unless you're sure it needs to be disabled. This article explains how ...

Your system will still work without the security feature, but what are the risks if you disable it?

Acer has fixed a high-severity vulnerability affecting multiple laptop models that could enable local attackers to deactivate UEFI Secure Boot on targeted systems. The Secure Boot security feature ...

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 ...