To me it seems such an fs should be immutable-first rather than read-only, i.e. let you create (by copying from another fs) files which just can't be changed (can be moved though) ever after. And such would be what I actually need as I have a lot of big and relatively redundant files which aren't meant to change (e.g. video files, picture originals, distro and backup disk images etc.).
$ mkdwarfs -i /path/dir -o image.dwarfs
[1]: https://github.com/mhx/dwarfs/blob/main/doc/mkdwarfs.md
As then, you can dedup per block, even with a sliding window. Nicest is that you can do it fully asynchronously, so real time appends are not slowed down.
Which are many.
Some with deduplicarion built in.
The same way we [used to] use specialized software tools to assemble an ISO 9660 image with our data, and then burn that image to a [single-session] write-once optical disk like CD-R.
...Or, in the author's case, when you have hundreds of directories that have only small differences compared to each other.
As other posters in the bigger thread mentioned, this is also very useful for big arcade game collections.