It is both.

Home Assistant created an OS for appliance like installations.

But there is also the docker images, repo packages (I know Arch Linux has it in the repo) and pip based packages too.

Home Assistant has a lot of moving parts, all the add-ons and extra user stuff. So they provide a docker image with everything you need, they also provide a full appliance install for easy setup.

If you did all that as a package install, you would complain about all the dependancies and if you didnt install the right version of something Home Assistant might not work at all

Before I understood Docker, I used to have HA installed directly on bare metal side by side with other "desktop" apps.

To be able to access devices, HA needs many different OS-level configurations (users, startup, binding serial ports, and much more I don't have a clue about). It was a giant mess. The bare OS configuration was polluted with HA configurations. Worse, on updating HA, not only did these configurations change, the installation of HA changed enough that every update would break HA and even the bare OS would break in some ways because of configuration conflicts.

Could this be managed properly through long term migration? Yeah, probably, but this is probably a ton of work, for which a purpose-built solution already exists: Docker. Between that and the extra layer of security afforded by dedicating an OS to HA (bare metal or virtualized), discouraging the installation of HA in a non-dedicated environment was a no brainer.

What do you mean with "Operating System"?

It is most often installed as Docker container, which isn't an OS, but just includes all dependencies to run. You still need an OS (like Debian) as host.

I installed it on Ubuntu server on my raspberry pi 4 and it took a couple months to fall over and become useless.

I've been running their OS since then and it has been absolutely rock solid. It's been 5 or 6 years now, all I do is add more devices occasionally and update it when it occurs to me.

If you have a life and you don't absolutely love tuning your OS for special purposes in ways that are already solved problems, the hass os image is the way to go.

Look into "install home assistant core" on https://www.home-assistant.io/installation/linux

You should ask this to your favourite distro packagers, not to the home assistant developers.

In any case, it is such a mess of dependencies due to load of optional packages, very active development, that continuously break dependencies on the package repo.

What advantage would bring have a most of the time obsolete distro specific repo? On a maintainer POV this is the typical use case for distro agnostic deployment, maybe flatpak, maybe docker.

It can also be installed using docker containers but that is more difficult to manage as you have to install every component manually.

It's true some things are harder to do in the container configuration; it's easier installed as an OS, especially integrations like Z-Wave, ZigBee, RTSP, Eufy, ESP, and so on. All of these require running other software, and in containers it's a fair bit of fussing with port and host OS device connections.

I've always run it in a container, without issue. It works fine, but I'm comfortable with the command line and LXC. That said, flashing an ESP hardware device and getting it connected to HA running in a container has so far defeated me, because I have to give access to the device in the configuration of the container before I run it, but the device flashing process itself is time limited and expects a process to be waiting on it when it is connected. It's a chicken/egg problem I haven't yet figured out which wouldn't be a problem if I were running the HA OS.

HA isn't the only software that just works better when it controls the while OS. Kodi is another that encourages users heavily to running it as an OS.

Regardless, it runs fine via

podman pull ghcr.io/home-assistant/home-assistant: latest

and there's a package in AUR that wraps the container up with a systemd service - it's as close to a bare package install as you're likely to get.

What's a little funny to me is that, despite that I've been running HA in a container for the past 4 years, I'm working towards getting a dedicated device and running HA OS on it. If we ever move out of this house, I'm not going to spend weeks going around replacing all of the hardware - smart sockets, lights, garage door opener, security, etc etc - with dumb devices; and for any of that to be worth anything, it's going to need a controller configured for it, which means, I'm planning on selling the HA server device with the house. For that case, I don't want anything but HA running on that device, and for that, it'd just be easier and smoother to run HAOS.

My advice is to run HA in a container until you are sure that's the direction you want to go, but not for so long that it's going to be a PITA to migrate to a dedicated server. But - hey, just IMHO - plan on running HAOS. If I knew then what I know now, that's what I would have done.

@rah There are tons of applications which can be installed via git clone and make instal using a isolated environment like pyenv or others. So I agree.
Now I get an Haos and docker thingy which is very restrictive on most types of installs. If installed different than the maker wants, no addons or more restrictions.