Optiplex 3080, no boot into Legacy Mode?

It uses later class of UEFI bios which has dropped legacy bios support. You cannot boot into legacy mode.

UEFI machines can have one of the following "classes", which were used to help ease the transition to UEFI.  To ensure backward compatibility, earlier class UEFI firmware implementation could support booting in legacy BIOS mode from MBR-partitioned disks through the Compatibility Support Module (CSM) that provides legacy BIOS compatibility.  Intel decided to phase out support for CSM in 2020.  3080 was released in 2020.

• Class 0: Legacy BIOS
• Class 1: UEFI in CSM-only mode (i.e. no UEFI booting)
• Class 2: UEFI with CSM
• Class 3: UEFI without CSM
• Class 3+: UEFI that does not have a CSM and has Secure Boot enabled

@smyaki 

Legacy booting was removed from cpu's after 6th Gen Skylake in 2015.

Older models like 3040 will still boot legacy and run XP or WIN7 32 bit.

7th gen and higher CPU's are 64 bit only UEFI only 3050 3060 3070 3080.

Which is why there are no XP or 7 drivers for newer Chipsets and CPU.  Nothing whatsoever to do with the release date of the 3080.

This was INTEL, Microsoft Decision where after 6th gen its WIN10 only and 64 bit only.  NO MBR,  NO CSM, NO 32 bit anything.

Windows 8 had Class 2.3.1 UEFI

Windows 10 after skylake 2015 has Class 3 UEFI without CSM.

This was discussed  as early as 2017 and finalized in 2020.

UEFI Brian_Richardson 2017

So there is no such thing as legacy boot on internal storage.

https://www.dell.com/support/article/en-us/sln309720

legacy was gone after 7th gen CPUS as well as 32 bit driver support  that's an INTEL Microsoft decision.

If you want to continue you will need to find a 6th gen or earlier model that supports 32 bit legacy OS.

"Windows 7 was designed 12 years ago. For Windows 7 to run on any modern silicon, device drivers and firmware need to emulate Windows 7's expectations for interrupt processing, bus support, and power states which is challenging for Wi-Fi, graphics, security, and more," "OEM's make customization's to legacy device drivers, services, and firmware settings, customers
have been told 7 is EOL. There will not be Windows 7 ongoing servicing."
There is no more support for Windows 7 and Windows 8.1 on the newest PCs past Intel's Sky-lake processors ALL next-generation processors will require the "latest Windows platform and no longer have 32 bit support orCompatibility Support Module (CSM)." All will be CLASS 3 UEFI.

https://www.dell.com/support/kbdoc/en-us/000125414/

Systems past 2015 sky-lake 6th gen are class 3 bios UEFI 64 bit only WIN10 only. If you need that get an older model like optiplex  790 7010 7020 7040 990 9010 9020 etc.