Seekable formats also allow random reads which lets you do trickery like booting qemu VMs from remotely hosted, compressed files (over HTTPS). We do this already for xz: https://libguestfs.org/nbdkit-xz-filter.1.html https://rwmj.wordpress.com/2018/11/23/nbdkit-xz-curl/

Has zstd actually standardized the seekable version? Last I checked (which was quite a while ago) it had not been declared a standard, so I was reluctant to write a filter for nbdkit, even though it's very much a requested feature.

It's not standardized as far as I know.