Are there still use-cases for ECL?
Digi-Key still has thousands of active part numbers. Most Popular are the analog input comparators with extreme speed and symmetrical slew and delay on outputs and also prescaler counters in the 4~5GHz range.

Complementary ECL has its speed and no spikes for egress ( both conducted and radiated) The low noise emission from complementary ouput signals that are perfectly matched cancel each other out and consumes constant supply current, unlike CMOS.

The 100 Series contains temperature compensation.