Modern Standby is a feature from Windows 8 onwards where a machine is still alive in a lower power state.
This is more or less like a mobile device waiting for a call etc.
in an elevated command prompt run:
and you will see that State 0 is not supported etc
it seems that AMD is behind the curve for power efficiency
Yeah... that's not right.
I mean it is correct that Windows will say that Standby (S0 Low Power Idle)., which is also known as "Modern Standby" as well as "Sleep" is unavailable.
But what's incorrect is that this only occurs on AMD Systems., you'll find identically Intel Systems will also say that S1/S2/S3 are "Not Available".
Why? Well because these part of the ACPI (Advanced Configuration and Power Interface) Standard., which what is supported is entirely dictated by your Power Supply and what has been enabled in the BIOS.
With this said ACPI Standby is a legacy feature that has yet to be removed...
Today CPUs (Chipset) can fully control Power Deliver from the Power Supply., and they also have a large number of useful P-States... including Sleep which is a "Special" P-State that Powers Down the "Cores" while keeping the I/O Chipset in an Idle P-State that keeps the Memory in a Low-Power, Low Clock Mode and actively listens to connected Devices (USB / Network) for "Wake Up" calls.
Sleep Mode first introduced in Windows 8 uses these features of Modern CPUs over S0., even on Laptops.
In essence Windows now has fully control over Power and Processor Settings without having to rely on the BIOS to handle such changes; thus making Standby S0 - S3 more or less obsolete.
You can of course enable Standby S0 if you REALLY want it., most Modern Power Supplies do actually support it... it's just disabled / set to S3 Only by Default.
Still how Windows handles things now is a better solution.