Start by playing with subvolumes and snapshots so you can get a feel for how they work. Once you've got that down you can break down your root filesystem into sensible subvolume chunks (/, /home, /var/log, /var/cache etc) so that you only snapshot relevant content during each update. I wrote a btrbk config at that point, tested it a few times and then wrote a pacman hook to fire it on install, update or package remove events and went from there.

Here's what I use to take snapshots - you'll need to write an appropriate btrbk config file for your subvolume layout but it's otherwise feature complete. https://gitlab.com/arglebargle-arch/btrbk-autosnap

Like I mentioned above, I haven't actually needed to roll the system back in ages but I get a lot of mileage out of being able to reach back in time and grab old versions of files for comparison.

Time shift is a lot easier if you're just starting out but it also requires a specific subvolume structure and isn't very flexible.

Edit: pro tip: don't make /var a separate subvolume from /, it's way, way, way too easy to roll one or the other (/ or /var) back without the other. If you do that by accident pacman's state becomes out of sync with the running system and everything breaks. Stick to splitting frequently rewritten data like /var/cache and /var/log off, leave /var itself in the root subvolume.

Timeshift, Timeshift auto-snap, and btrfs in the grub menu to have your snapshots there, too. Auto-snap takes a snapshot automatically whenever you upgrade or install some packages.

Oh totally fair, it doesn't have a huge maintainer base for sure. But it'll never be anyone's daily driver if no one knows about it.

Well to be fair I think it's because they aren't trying to be NixOS. You could leverage those arguments against any distro that's trying out an immutable flavor. Which is mostly accomplished through btrfs features.

I agree that Nix/NixOS does a lot more and it's a genuinely impressive and paradigm shifting project but it does break with traditional Linux layouts and thinking in a way that immutability doesn't necessarily have to do.

You could also make the same argument with the systemd and non-systemd crowd.

Either way I look forward to the future of both immutability projects and NixOS. I feel like both areas still need a bit of work but they're both really exciting fields.

Which packages?

1. Check nixpkgs unstable, they might have been added in the last few months before stable release
2. Try steam-run, it will run binaries like you're in a normal distro

I ended up packaging the thing myself, actually. The best part is my pull request was approved and I was able to contribute my work

In that case, couldn't you just use something like btrfs snapshots + Timeshift to pull this off?

Just add rescue to kernel options (if you use GRUB, press e to edit it for the current boot) and it will boot into console from which you can do downgrade.