For my company:
Mandatory in Africa
Recommended in South America
should consider for NATS
I was under the impression that SLOP was very helpful concerning opposite direction traffic, and not so much for same direction traffic.
The rate of closure for nearly same direction traffic gives me more time to detect, analyze, and take preventive action compared to opposite direction traffic.
There is always that occasional jet that did not show up on TCAS. SLOP proves its benefit when he goes by 2 or 3 miles off the left wing.