Which is unfortunate, because the 2015 models make great Linux machines.
The introduction of the T2 chip made proper Linux support much harder to achieve.
Oh, and Apple's NVMe interface is non-compliant. This is widely reported as Apple locking Linux out with the T2 chip, but that's not really true. The T2 chip will by default prevent unsigned kernels from reading/writing to the SSD, but this can be disabled.
Even if it's disabled, the controller is not standards compliant, and Linux won't see the underlying block device. I saw some diffs floating around on github a few years ago that fixed it, but I don't think it was ever mainlined.
Basically, post-2016, Apple seems to have incorporated even more custom (and undocumented) hardware that running alternative OSes on them is basically impossible. Windows works because of the Apple-provided HAL + drivers for WinNT.
As far as I know it’s only available on Apple OSes.
Your claims about "dismal then and only got worse" are unfounded. The repository you refer to is still active. https://github.com/Dunedan/mbp-2016-linux If anything, activity has slowed down in these threads because it was figured out how to make it work.
Even among people who run Linux on these MacBooks, the general recommendation is to keep a macOS partition around for stability. Some of the value you get from any Apple computer is in the software. If you intend on instantly installing Linux or Windows as your only OS, this probably isn't the computer for you. But if you want to or have to use Linux sometimes, these T2-chip Macs can do it.
As I'm said repo owner, let me chime in here quickly to shed some light on that.
I used a 13" MacBook Pro 2016 for 3 years with Debian as my sole machine for work. When ordering it back in 2016 I wasn't sure how difficult it'd be to get Linux properly working on it, as at that point it was only known that it's possible to boot Linux, but nobody had figured out even such basics like support for the integrated input devices or the NVMe SSD yet. However as I was using Linux on Macs since 2006 I figured it'd be somehow possible to get it to work for me.
Fortunately I wasn't the only one serious about running Linux these 2016+ MacBooks, as I have very limited knowledge of the required lowlevel programming skills. What I did was to provide and moderate a Github repository (https://github.com/Dunedan/mbp-2016-linux) as a central place to document and discuss of the status of hardware support for these MacBook Pros, some little patches and lots of feedback and bug reports. A big shoutout to all contributors who did an incredible job at reverse engineering, implementing and upstreaming drivers for various components! That's quite an achievement for such a complex device with no public hardware documentation at all!
After a while it turned out that support for certain components would be rather difficult to get working flawlessly. As an example, even at the end of the 3 years I used the MacBook Pro, I had to use an external adapter to be able to use WiFi. With that in mind I started to reconsider why I bought Apple products: I bought them because of their superior hardware quality. But if I'm not able to use the hardware as intended, what's the point of paying a premium for Apple products? And let's just not talk about the butterfly keyboard or the horrible thermal management. So when it came to replacing my MacBook Pro, I decided to go with a Lenovo Thinkpad X1 Carbon instead. It's not perfect, but I'm way happier now than I ever was with the MacBook Pro 2016, as the hardware just works.
As I don't own any 2016+ Apple device anymore, the help for further Linux support I can provide is limited, but I didn't stop bothering at all! I'm still actively managing said Github repository, but activity in general has significantly dropped there over time. Either the devices work well enough for other people now or they also replaced them with non-Apple hardware.
I bought a 2017 MBP hoping the situation would eventually improve but it never did, so I never got around to installing Linux. I'm expecting it'll be even worse for these M1 systems.
It is a shame, it's not something I ever really did (or not for long, for a period I do recall having Arch on my 2013 Air) but I like the idea - I like Apple's hardware, just not the software.
So.. depending what you want to do on these older machines, my point is that this may be the least of your worries.
^(though I think it's fine, because it's the reverse that would be a problem? Bad news for 'hackintosh' if all supported versions of macOS can expect secure boot hardware, I think)