I am unsure if the developer intends to work further on it. Therefore, users may also be interested in several pull requests that are, at time of writing, available at https://github.com/henn/qubes-app-split-ssh/pulls:
1) Adds the autostart .desktop file which starts the ssh-agent in the vault
2) Improves the socket permissions in the client AppVM
3) Adds the notification in the UI, like Split GPG, when the agent is accessed
4) Adds the ability to set a 'timeout' so as not to be prompted on every SSH request but (say) only if 10 minutes has passed without any SSH requests having been made. (similar to Split GPG behavior)
Thanks to the developer for progressing this effort. It's working well for me :)
Cheers
All except for the last pull requests have been merged (need to spend more time studying the last one).
Jason