All Linux distros are roughly the same under the hood. They have a version of the Linux kernel, a service management system, a basic set of packages and preinstalled software, and some kind of UI. How those pieces are put together realistically doesn't change much. What distro maintainers then do is update the packages and kernel and make sure they work together.

Specific design distros like the one for scientific modelling is probably based around having a lot of data processing stuff built in. Unless they had a specially modified kernel (which is rare, but possible if they had to integrate special lab data collection hardware support), I see no reason why you couldn't just recreate the same package structure on a modern Linux 6 kernel and have it be a spiritual successor to the original distro.

A "distro" is basically just:

• a precompiled kernel
• some preinstalled software

Idk anything about Poseidon but,

is it easy enough to replicate just by downloading the relevant packages?

The answer to that is yes. Just pick a base you like (eg debian, fedora, ubuntu, ...) and install the software you need. You could automate it with some simple scripts, or be fancy and write a Butane config to preconfigure a base Fedora CoreOS image. IIRC, the Omarchy distro is just the former, not even a proper "distro".

If you want to create your own "proper" distro that other people will want to use, there's a lot more that goes into it: updates, builds, tests, deployment, patch sets (because you'll inevitably need to patch various components for compatibility), bug reports, some kind of governance structure...

It's a whole software development thing. If you just need a customized platform for your buddies/workplace, customizing an existing distro is the only reasonable choice. I'd suggest looking into bootc and ublue if you need more than a simple post-install shell script.

Poseidon also had daughters.

To me resurrecting and maintaining a distro would be a bit harder and more work than to just installing the software you need. Could be a rewarding learning experience though.

You could always try to contact the original maintainers and see if they have any advice. Maybe why the project was abandoned.

If you are ready to build your own packages and host your own repos, go for it. You should have some form of automation to at least get security updates, and it will take some recurring maintenance time no matter how you go about it. I haven't looked closer at Poseidons repos but I think if this is a good idea or not depends a lot on the state of their build/packaging/distro code. It could be just a matter of cloning the repos, changing a few config parameters and running a couple of commands, or it could mean significant work, depending on how well it is engineered. You could start out and if you find yourself digging to deep in the process of getting a build, back out and reassess.

Kind of like others suggested, there are lighter options. All the big dists should have some form of build tooling to make kickstart/preseed/spins/whatever they call it where you can prepare a custom install ISO with your own set of packages. There are tools like packer and Nix that you can use either as part of building OS images or just script to run on a clean base installation of some dist. You can make an ansible playbook to automate setup and have that run by cloud-init. You could make a shell script to automate the installation of packages and setting up of environment.

If you want to do it, use ublue. You don't have to learn how to create a distro. You just use a ublue base and add the packages you want. It's incredibly easy

maybe a distro like this is less needed now that we have Flatpak, AppImages, and (yuck) Snap

you mentioned 2018, I think that's about when Discover added support for Flatpak and Flathub

What made this distro distinctive, and for what features?

There's not much info about it that I can find. Do you have more info about what was specific about it? Wikipedia says:

Poseidon Linux was aimed at the international scientific community, it came with a range of applications for fields such as GIS, 3D visualization, and statistics.

So it sounds like it just came with some apps pre-packaged (which is what most Linux distros do). If that's the case then yes, you absolutely can. How easy it is depends on what the specific apps are and their installation methods.