SERVER-109946 has modified the index descriptor to distinguish between

• Sparsity set by the user ( called isSetSparseByUser ) which replaced the old "isSparse"
• Implicit sparsity ( called behavesAsSparse ) which indicates that either

• • The index is set sparse by the user or
  • The index type is implicitly sparse (i.e it behaves as sparse even if the user sets sparse = false).

The old "isSparse" method had a confusing name, and might have led to previous developers to think the index "behaves as sparse" when in reality it didn't. 
We should review all the usages of the old isSparse (now  isSetSparseByUser) to determine if "behavesAsSparse" is more appropriate. 

As part of the PR, we have identified some places where the new helper might required to be used, such as here , here  or here.