Per following note from seny.kamara@mongodb.com

No it does not have to be a power of two anymore; but we will be opinionated setting it and provide guidelines to customers

and per updated specifications, where sparsity must be at least 1 (otherwise division by zero happens):

createCollection should adopt a new definition of sparsity being a number from 1 to 4. Number 4 is chosen arbitrarily and could be later updated to a larger number. We must limit that number to a reasonably low value such that we don't find ourselves in situation where minCover would run out of memory because it would consist of purely leaf nodes