That was never an officially supported API end point and in fact it’s use was actively discouraged. I don’t know that it has since been removed but if so they would not have announced it since it was never supported in the first place.
There are several alternative ways for one OH to see and control another one though.
- Remote openHAB - Bindings | openHAB
- MQTT Event Bus [4.0.0.0;5.9.9.9]
- Webhook HTTP binding (I’m not 100% sure this works with 5.1, but it seems to install at least)
The first option is the officially recommended way to implement this use case.
Note, the Tutorials and Examples section is a place to post tutorials and examples, not a place to ask for one.