But you can't do that on a live system as you can with Windows or macOS. Not a problem for pre release upgrade perhaps. But I'm so missing this feature from macOS.
You can if you're using LVM. Take a snapshot of the logical volume your system is on, then run `dd' against the snapshot, as it's essentially a frozen point-in-time.
I've used this trick many times in a live, rw environment.
You can snapshot the filesystem if you're using BTRFS, ZFS, or another Copy-on-Write filesystem.
Depends on your filesystem. For example, I certainly can as I’m using btrfs. I’m also using Timeshift for easy management of snapshots. As others have mentioned, there are other choices too like Snapper that all work well.