I just purchased NUC9VXQNX and I needed to install specific OS which needs Legacy boot as per installation instructions. Unfortunately I am unable to do so in this model. I checked Intel guidance here https://www.intel.com/content/www/us/en/support/articles/000032529/intel-nuc.html
But There is no option to disable modern standby in this model. I see an option named "Native ACPI OS PCIe Support", which is apparently the same thing, but it is greyed out in the BIOS, and I am not allowed to disable it.
Looks like someone incorrectly marked the OP's response as SPAM. I will answer it anyway.
You are out of luck; Legacy Boot is not supported on this platform. For more information, consult: https://www.intel.com/content/www/us/en/support/articles/000057401/intel-nuc.html.