When attempting to modify the drops of a mob spawner to allow it to drop an item so long as it is broken with silk touch, it does not work. I even tried simply making it drop an item regardless of how it was broken. and still no drop was spawned. I attempted using the same configuration on another block (in this case glass) and it dropped the spawner as expected. So the only conclusion I could come to is that it was the spawner itself not being responsive to the data pack.

Data pack I used to test spawner drop and replicated spawner drop on glass is included.