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.

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

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

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.