Bios Mode on Equipment Legacy or UEFI Information is available in msinfo32.
This is the custom inventory rule that I use.
name is Inventory: Firmware Type
ShellCommandTextReturn(powershell -executionpolicy bypass -command "$env:firmware_type")
I then have a smart label named: PC-Legacy BIOS I filter in win 10 so my servers aren't included I'll handle those manually.
Inventory: Firmware Type contains Legacy
OS Name contains Windows 10