It seems like a growing number of things once referred to as Linux distributions are now referring to themselves as operating systems. If the kernel is Linux, and the user-space is GNU, what makes this a distinct operating system from, say, SUSE, or Arch?
The userspace is so diluted now that it’s basically flat out wrong to say it’s just ‘GNU’, I mean Systemd is probably an even bigger a part than GNU is now, and we’ve long had things like OpenSSH from BSD as pretty core parts of the system, and we’re not going to start calling a distribution ‘Kubuntu Linux/Systemd/GNU/BSD/KDE’ or whatever…
Basically about all something needs to be to be called an OS is a kernel and at least one userspace program that does something useful, so I’d definitely say every ‘Linux distribution’ has always counted as an operating system in itself (so ‘Linux distribution’ is just a specific subset of ‘operating systems’).
I like to thing of GNU/Linux as Linux with glibc. There’s software that only runs with glibc (eg: steam), and software that runs with various libc (eg: Firefox).
I’m not sure that it’s a widely accepted definition, but it’s often useful to describe what a software depends on. Does it require _just_ Linux, or does it also require glibc?
A distribution focuses on the distribution part (eg: a package manager, repositories, etc).
Some distributions are operating systems (eg: OpenBSD, ArchLinux, Debian). Some operating systems are not distributions (they don’t include a mechanism to pull packages. Eg: windows, macOS). Some distributions are not operating systems (eg: homebrew, Flatpak).
Tails focuses on the operating system side of things. It’s focus isn’t on package distribution and letting you install things, but on downloading a usable OS image. It’s still a distribution, but that’s more of a technicality.
I'd say the reason for that is marketing, or branding, or positioning the product, which are, as you wrote, essentially Linux distributions.
I find that even combinations that are supposed to be very similar (Linux kernel, same DE, same repos) can behave differently, and I guess this is because of how the distro maintainers set up the different parts and integrations in the system. So in this way, my MX Linux box is different from my Debian+KDE box.
