The RelBuilder#groupKey(ImmutableBitSet, ImmutableList) API is marked for removal before 2.0.
I would suggest to remove it ASAP due to the fact that it clashes with the new replacement API.
Leaving it as is creates ambiguity and requires everybody to upcast to Iterable if they want to use the new API.
Removing it on the other hand does not seem to affect much backward compatibility since callers will automatically use the new API with no changes required in their code.
The main difference is that the deprecated API allows groupSets to be null but if we care so much we can also make the new API to be more permissive.