What demonstrates ISL use is service availability in the middle of oceans, far from any ground station.

https://www.starlinkflights.com/how-to-check-starlink-flight:

"Unlike older geostationary satellite systems that had coverage gaps and high latency over remote oceans, Starlink's low-earth orbit constellation covers the entire globe. Qatar Airways, United Airlines, Alaska Airlines, and airBaltic all offer uninterrupted Starlink on long-haul international routes."