> Does your hardware vendor offer long term support for Linux?
This seems muddled. If the CNC manufacturer puts Linux on an embedded device to operate the CNC, they're the hardware manufacturer and it's up to them to pick a chip that's likely to work with future Linuxes if they want to be able to update it in the future. Are you asking if the chip manufacturer offers long-term-support for Linux? It's usually the other way around, whether Linux will support the chip. And the answer, generally, is "yes, Linux works on your chip. Oh you're going to use another chip? yes, Linux works on that too". This is not really something to worry about. Unless you're making very strange, esoteric choices, Linux runs on everything.
But that still seems muddled. Long-term support? How long are we talking? Putting an old Linux kernel on an embedded device and just never updating it once it's in the field is totally viable. The Linux kernel itself is extremely backwards compatible, and it's often irrelevant which version you're using in an embedded device. The "firmware upgrades" they're likely to want to do would be in the userspace code anyhow - whatever code is showing data on a display or running a web server you can upload files to or however it works. Any kernel made in the last decade is going to be just fine.
We're not talking about installing Ubuntu and worrying about unsolicited Snap updates. Embedded stuff like this needs a kernel with drivers that can talk to required peripherals (often over protocols that haven't changed in decades), and that can kick off userspace code to provide a UI either on a screen or a web interface. It's just not that demanding.
As such, people get away with putting FreeRTOS on a microcontroller, and that can show a GUI on a screen or a web interface too, you often don't need a "full" OS at all. A full OS can be a liability, since it's difficult to get real-time behaviour which presumably matters for something like a CNC. You either run a real-time OS, or a regular OS (from which the GUI stuff is easier) which offloads work to additional microcontrollers that do the real-time stuff.
I did not expect Windows to be running on CNCs. I didn't expect it to be running on supermarket checkouts. The existence of this entire class of things pointlessly running self-updating, internet-connected Windows confuses me. I can only assume that there are industries where people think "computer equals Windows" and there just isn't the experience present, for whatever reason, to know that whacking a random Linux kernel on an embedded computer and calling it a day is way easier than whatever hoops you have to jump through to make a desktop OS, let alone Windows, work sensibly in that environment.