Mathcad 15 still misses (and ever will be missing) quite some features which where demanded for long times by its users and Prime essentially is missing a lot of features we already are used from Mathcad 15 and below while adding only very few additional benefit (like mixed units in arrays).
Maybe we would need along with the literal and the vector index a third subscript denoting partial derivative to make the confusion as of which subscript to use complete and maybe Prime will offer a better notation for partial derivatives one day.
If you solve a PDE using a solve block with PDESolve, the literal index has to be used to denote partial derivatives anyway and so it follows the usual subscript notation.
For other (documentation) purposes you may define it yourself and hide the definitions in a collapsed area. You can't hide that area completely making it invisible as in MC15 but sufficent enough so its out of the way. So at least you can duplicate the subscript notation from the textbooks.
