Sierra security vulnerabilities

Perhaps because the Pegasus attack was oriented toward security flaws in functionality only in macOS Catalina and Big Sur, that does not exist in Mojave and older versions of macOS. Additionally, macOS Sierra has be out of Apple's support cycle now for two years, and you wouldn't be receiving any security updates for that reason alone.

You are welcome. If you remain on Sierra, just switch to Firefox 92 (current) for the latest browser security that is no longer in your Safari version. Big Sur only supports 64-bit applications, printers, and scanner drivers and that is not a leap you want to take without looking at that situation first. The free Go64 application can help you see what 32-bit applications are currently installed on Sierra.

When you decide to upgrade, the How to get old versions of macOS will guide you on the hardware requirements for an operating system upgrade, and a relevant link to the appropriate full installer in the Mac App Store.

User wrote " new security hack " and what might that be ??

Sierra - very doubtful Apple will issue any more Security Updates.

I have three older iMacs, all on OS Sierra 10.12.6, why won't Apple even acknowledge there's been a hack ?