This is a strange one that I discovered on my new build/upgrade. Please excuse my rant. This is the relevant hardware:

Intel Alder Lake i7 12700K
Asus TUF Gaming Z690 Motherboard(DDR4)
See:
https://www.amazon.com/Intel-i7-1270...07&sr=8-3&th=1
https://www.amazon.com/ASUS-TUF-D4-m...s%2C136&sr=8-3

Haven't done a build since 2018 and things have changed - components are obtainable but in short supply and prices have risen considerably. Anyway, got the board, cpu and ram in the case and was going to test with the intel integrated graphics to see if it would POST and the temps were OK before adding my graphics card and storage. POSTed fine and went into the Advanced section of the UEFI Bios to disable secure boot and enable CSM which is Asus's name for their compatibility mode allowing booting of non-EFI operating systems. Disabling secure boot was fine but when I went into the section where you enable CSM the entry was visible but greyed out and you couldn't select it.

Checked the motherboard manual to see if I was doing something wrong and there was scant information on their BIOS other than this warning in bold type:
So they'll sell you a motherboard; expect you to install it; but you need "trained service personnel" to configure the BIOS!
There was a QR code which you had to scan on your phone to get a link to actual motherboard documentation where I found this:
Sure enough, when I installed my graphics card, I could enable CSM and all BIOS references to the onboard intel integrated graphics were gone. I removed my graphics card and powered up again and the CSM option was greyed out and the intel integrated graphics BIOS references were back.

My only question is whose bright idea was it to code their UEFI like this, i.e. make access to the onboard integrated graphics dependent on restricting booting to EFI enabled operating systems! A quick internet search revealed that I was not the only one pulling out my hair over this.

This was an upgrade from my old i5 Haswell cpu and board and I had planned to just throw in my NVME drive on which I had no fewer than five linux distros installed, all of the legacy BIOS variety, in my new build. If I didn't have a graphics card and had to use the intel integrated graphics on my new build, I would have been forced to reinstall all of them in EFI mode.

Perhaps your only question should be why are you still using "legacy mode" ?. They surely have the right to decide whether they will spend their development dollars supporting obsolete arrangements.
Maybe it's time you caught up with the world.

The update to the linux systems can be done in situ - I imagine a chroot from a UEFI aware liveUSB should work but I've never needed to try.

Microsoft

Or you can do like I do, turn off UEFI and refuse to use any software that requires UEFI.

On one of my computers I have to change two settings to get between the two. One is the normal csm but I can't change it if some video setting is holding it. Forget the exact name but that is what your issue most likely is.

There is a way to convert to uefi without reinstalling using a live iso.

Asus support told me Intel eliminated CSM support in 5xx and newer chipsets and/or iGPUs when using 11th generation or newer GPUs. I don't remember which if it was "or". My B560 chipset Asus B560M-A using i5-11400 CPU's UHD 730 graphics won't let me enable CSM.

I can't think of any good reason not to use UEFI on a fresh installation, but I do understand need to migrate MBR partitioned HDD or SSD to new motherboard when the old motherboard poops, or hardware upgrade is merely desired. Of course, disk migration to foreign motherboard is not something doable with Windows, so doesn't need to be supported by the manufacturers.

UEFI is a good thing.

This was their stated goal from the very start. Most AMD systems still have CSM. For now.It's an extremely "good thing" for Intel, Microsoft and their biggest customers, which are not end users, but the media industry and proprietary software vendors: It finally enables them to fully control all privileged software running on PCs, and keeps plebs like you and I from installing "unauthorized" operating systems or low-level drivers that might circumvent DRM or license enforcement mechanisms.

(If you can think of a single advantage offered by UEFI that couldn't be accomplished just as easily using a standard BIOS, I'd like to hear about it.)

Anyway, UEFI marches ever onwards:

1. UEFI is introduced, CSM is supported, Secure Boot can be disabled
2. Windows introduces UEFI support, Secure Boot is optional
3. Hardware manufacturers remove CSM from BIOS, Secure Boot can still be disabled
4. Windows removes "legacy" (CSM) support, requires Secure Boot <-- you are here
5. Hardware manufacturers remove option to disable Secure Boot
6. Hardware manufacturers remove ability to add 3rd party keys

The UEFI BIOS loads a startup file from a VFAT ESP filesystem, instead of being limited to loading disk boot sector binary code. Thus rearranging disks is unencumbered by rescue booting to "reinstall" a bootloader. Simple copying of a file or two or three, if that much, maybe accompanied by a little plain text editor tweaking, gets the rearrangement job done with simple, common, easy-to-use tools, with efibootmgr as an optional enhancement.

Optionally, the UEFI may look for alternate ESPs and ESP files to present in its BBS menu, while a legacy BIOS still can only load boot sectors.

Legacy/MBR booting was long overdue for a better way.

How often does one "rearrange" disks, and how is the procedure you outline any easier than using more or less the exact same tools from a bootable USB stick?

The only main differences are that with UEFI, you have an entire OS and some tools in the BIOS itself (complete with security vulnerabilities), and you have to store the boot files on a partition with a FAT file system, which is notoriously brittle.
And the boot sector points to a boot loader, which may offer a boot menu and command line tools. If you need that, GRUB will do the job quite nicely.
The advantages of UEFI are questionable at best, and constitute a Trojan horse at worst. I'm still installing all servers using CSM/MBR on GPT drives, and will continue to do so as long as the BIOS allows it.

(After that I expect we'll all be running Linux in a VM on Windows anyway, in which case the choice of boot mechanism is entirely irrelevant.)

For others, I can't answer. Here there are many computers, comprised of many pieces that are often used as troubleshooting tools. Since I don't use Windows, I can shuffle disks around with abandon and occasional little touchup via a normal boot, without any chrooting or grub scripts.

On an average boot, it gets read once, written none. I have no problem living with that, or the trivial amount of space an ESP requires.

Indeed. So well in fact, that all my non-UEFI systems not booting OS/2 are still booting using Grub, not Grub2. Grub2 is only present on all (but one) of my UEFI systems.

So questionable that I only have UEFI-capable one that is not UEFI configured (my first, an MSI Haswell), which was a disk/RAID migration to newer motherboard from a much older gone flakey. All 7 capable that were since acquired (1 Mac, 1 Asrock, 1 Gigabyte, 4 Asus) I'm happy to have UEFI in control of.

Microsoft has created UEFI to build a moat around their monopoly. When we reach step 4 some hardware manufacturers will begin to offer machines that do not have UEFI. At that point Microsoft will be imprisoned within their own moat and their market share will begin to fade away.

I worked within the framework of the IBM monopoly from 1967 through about 1995. The scenario I described in the previous paragraph is what happened to the IBM monopoly. Over time IBM's moat turned from keeping IBM's customers from escaping to preventing IBM from attracting new customers. Beginning about 1990 IBM slowly faded into irrelevance. Now IBM is run by a group of MBAs who think that good management is to continuously massage the books so that they show increasing profits in spite of continuously declining revenues.

As Kurt Vonnegut would say, "And so it goes."

No, they won't, because they will be bound by the UEFI specs as dictated by Intel.

The difference between this scenario and the IBM debacle, is that a) this is not (just) a Microsoft issue, and b) there's no customer base demanding non-locked hardware. Hardware running ARM Windows is already required to be locked down with Secure Boot, and no-one has made any serious complaints.

The thing that's hidden if you want to run away from UEFI is this: You need an MBR disk formatted with fdisk, or UEFI applies.

When I last bought, it was 2012/2013 which was a very bad time for UEFI on linux. Now elilo, rEFInd, & grub all handle it. Now there's HOWTOs aplenty. Do like me - reluctantly embrace it, as it's part of the present.

EDIT: What does CSM stand for, anyway?

Nope, you don't. Not even for booting. I haven't been using MBR for ages.