Slime mold aggregation typically consisted of identical clones - the highest level of relatedness it is possible to get. The ant and termite families that group selection enthusiasts loved got a long list of nice quantitative predictions from kin selection theory. The evolution of chickens that supposedly counted as evidence for group selection was explained neatly by kin selection: the chickens were couped up with their close relatives.
It was argued that group selection was needed to explain cooperation between strangers who were not genetically related. However, this line of argument ignored cultural kin selection - much cooperation between strangers could be explained neatly in terms of shared memes - rather than shared genes.
There were other factors that explained cooperation too: including reciprocity, reputations, virtue signalling and the Allee effect. It seemed as though group selection was trying to take credit for the moves of other theories which were already well-established.
Was there anything group selection was needed for? Fortunately, this question now seems to have been answered...