The reason they scrapped Home on iPod way back when was due to the fact that Hitachi's 1.8" disks was very slow and wasn't made to support the load such a feature would demand. It worked though, and I tried it during the beta, but it was very slow.
Some functionality remains today. If you attach a disk with user homes on, you are offered to log into those accounts too, besides local users. There's no straight forward way of syncing and moving home folders onto an external disk though.
In this day and age, Apple could probably offer Home on iCloud%u2026 They have all they need for that. One could probably even boot from iCloud since Netbooting works over HTTP. But it would be comparably slow, and probably prone to errors and data loss.