The mini-relay has been removed from the circuit a while back (probably more than a year back). If you want to use a relay, you can get a 24VAC relay and connect it to any zone like a sprinkler valve. We have tested several 24VAC relays that work fine with OpenSprinkler:
https://opensprinkler.com/using-24v-ac-relays-with-opensprinkler/
Also, if you are looking to control powerline devices, you can use an RF transmitter in conjunction with RF power sockets. RF station is now a standard feature in OpenSprinkler firmware, and OSPi circuit board has pin headers that you can directly plug a RF transmitter into. This feature is explained in this blog post:
https://opensprinkler.com/opensprinkler-firmware-2-1-1-new-feature-control-remote-power-sockets/