"file ./live/filesystem.squashfs failed the MDS checksum"

I burned the 2.9.4 ISO to a DVD.  Because my DVDROM drive went bad recently, I bought a replacement.  Amazingly, I found an exact match for my Dell Optiplex 960.  It plugged right in, passed diagnostics and successfully booted from DVD.  I went a burned it again.  Same problem.  Is there a known problem with the release ISO, or does Windows 11 ISO burn just not work?

If there was a "known problem" it would have been fixed pretty quick.
The guys that build the ISO test them, they aren't just built and thrown out into the wild, things just don't work that way.
Usually in these kind of issues the first port of call should be to check the download as tommy suggests, rather than just assume it's a problem with the way the ISO has been built.

Is there any reason you cant boot from a USB drive ?

Okay. I am using Rufus but it does not like the ISO. The complaint is "the ISO you have selected contains a UEFI bootloader that has bween revoked. and that will produce a "Security Violation" screen. Rufus suggests a more up to date ISO. linuxCNC 2.9.4 is not current enough?

I got past that. It attempts to boot from USB drive but this is as far as it goes.  See attachment. 
 

That's a windows splash screen, not a splash screen when booting a Linuxcnc install disk.

Download this program and install, after reading the whole page.
win32diskimager.org/

When you insert the USB stick you want to make a Linuxcnc bootdisk don't let windows do anything if it cant read your USB stick, windows does not be able to read the USB stick.

Run the Program you just installed.
Select the image, the image you have confirmed the md5sum is good, if not download it again and check again. I'm not a windows user so you'll have to search how to check the md5sum under windows. Also select your USB drive, do not select a hard disk.

Write the image to the USB stick.

If windows complains about not being able to read the disk DO NOTHING. Everything is ok.

Reboot the computer and select USB boot. If you are going to install make sure you have enough space on your disk or have an unused disk available for Linuxcnc.

I recommend this tool as all it does is copy form the image file to the usb stick byte for byte. It doesn't try to be fancy, it does exactly what is required to create a bootable USB Stick from the image.

Unfortunately, "The code execution cannot proceed because Microsoft.UI.Windowing.Core.dll was not found. Reinstalling the program may fix this problem." It did not.

Trusting CoPilot, I am burning the ISO to USB again with MBR partition scheme.

That did not boot. I am trying to boot from DVD again. Is there any upgrade path from 2.7.15 to the modern versions that does involve booting an ISO? This desktop is apparently a legacy BIOS.

The image needs to be written to the USB stick byte by byte. The ISO format does not have a partition scheme.
I would have looked into why the program wouldn't run

A properly written USB stick for booting Linuxcnc does not have a "partition" scheme.
I think Belana Etcher has a "raw mode" that writes the image file to the USB stick as win32diskimager would.

Oh hang on you have a working Linuxcnc install ?

If so you can use that to make the USB stick.

This is a very comprehensive article on how to do that
www.cyberciti.biz/faq/creating-a-bootabl...k-on-a-debian-linux/

Or you can search for WSL dd to USB.
WSL is windows subsystem for linux

learn.microsoft.com/en-us/windows/wsl/install