Update: Apparently it’s possible to tell the kernel to mask _OSI entries; no git revert needed.
After adding "acpi_osi=!Windows 2020" to the kernel cmdline args:
sudo grubby --update-kernel=vmlinuz-5.16.8-200.fc35.x86_64 --args='"acpi_osi=!Windows 2020"'
(edit: or sudo grubby --update-kernel=ALL --args='"acpi_osi=!Windows 2020"' to make it the default going forward)
and rebooting, I can now get into s2idle sleep with the power button. So it seems that telling the Insyde BIOS (3.07 anyway) that we “support/are” Windows 2020 makes it do Bad Things.
Background:
kernel docs: ACPI _OSI and _REV methods — The Linux Kernel documentation
and this for the “negate entry” hint: Linux: ACPI: Fix problems with Suspend, Resume, and Missing devices using acpi_osi=