In an action brought to enforce a provision of this chapter, such provision is not to be deemed unenforceable because of its inclusion in a section of this chapter requiring a State plan or specifying the required contents of a State plan. This section is not intended to limit or expand the grounds for determining the availability of private actions to enforce State plan requirements other than by overturning any such grounds applied in Suter v. Artist M., 112 S. Ct. 1360 (1992), but not applied in prior Supreme Court decisions respecting such enforceability: Provided, however, That this section is not intended to alter the holding in Suter v. Artist M. that section 671(a)(15) of this title is not enforceable in a private right of action.