Their website https://github.com/jdholtz/i3-restore/tree/master Looks like a pretty vital feature. Thank you very much.
Uh, this looks like a separate package? Why would this be part of i3 itself?
Because this is useful, like a plugin for i3.
It looks nothing to do with i3 in that sense. It's just separate software, designed for use with i3. It's versioned and released separately, and you have to manually add it to your config.