As Revision - Super User explains, multiple types of sleep states exist. Does Plasma control this? If so, which does it utilize? If it does not control this, what does? (The OS?)
482783 – “Restore previous status” doesn't enable Bluetooth at boot (and exiting sleep, sometimes). was what made me think to ask, because it might have an effect upon how certain hardware reinitializes.
i believe the kernel controls this but your hardware/firmware will ultimately dictate which states are available to you in the OS.
my PC only supports S3 and S4 (which the firmware describes as “deep”) and none of the lesser modes so using “sleep” from my kde desktop suspends the OS to disk the same as “hibernate” does… it goes to disk rather than just memory because i have enabled the “deep” option in the firmware.
The kernel queries your motherboard for sleep preferences via ACPI by default. Changes to your BIOS/UEFI settings should work just fine.
If you wish to change it but cannot manipulate it in your motherboard settings, the mem_sleep_default kernel parameter can control which mode the kernel will choose.