To allow other services or programs to depend on a sshuttle SSH tunnel, the sshuttle systemd service should use notify start mode.

This is actually recommended in the sshuttle documentation:
"If you are using systemd, sshuttle can notify it when the connection to
the remote end is established and the firewall rules are installed. For
this feature to work you must configure the process start-up type for the
sshuttle service unit to notify"
https://raw.githubusercontent.com/sshuttle/sshuttle/master/docs/requirements.rst

Currently the service is considered ready when the process has been started, but that does not mean the SSH tunnel is ready so other service or programs depending on it have to implement retry loops to work around this.

The only change needed to fix this is this one line patch:

--- sshuttle.service.orig 2017-10-22 20:50:01.000000000 +0200
+++ sshuttle.service 2017-10-22 20:51:37.963310698 +0200
@@ -2,6 +2,7 @@
Description=Create a transparent proxy over SSH

[Service]
+Type=notify
ExecStart=/usr/bin/sshuttle --dns --listen 0.0.0.0 --remote sshuttle_tunnel -s /etc/sshuttle/prefixes.conf -e "ssh -F /etc/sshuttle/tunnel.conf"

[Install]