Quirky has returned!
I created Quirky GNU/Linux in 2010 and the last release was in 2018. I then notified Distrowatch that the project was discontinued.
Quirky was a full installation, occupying an entire partition, the same as most GNU/Linux distributions. It did not support squashfs (SFS) files, nor did it use aufs or overlay fs layered filesystem. So it did not have "run in RAM with a save file"; what it did have was strategies to minimize flushes to the storage media, to help prolong the life of flash media.
There are some guys active on the Puppy Forum who would have fond memories of Quirky. Well, Quirky is back, though this is "Quirky on steroids". And very highly experimental.
Yes, the features are as described above; however, major new features are added. Quirky can be installed frugally, that is, be in one folder in a partition used for other purposes -- but it has to be a btrfs partition. There is a snapshot and potentially rollback/roll-forward capability, like we have in EasyOS and the pups. The filesystem has read-write compression. There is per-folder encryption.