This hasn't been true for a long time and DX12 doesn't really change anything here. DX11 was already able to handle very large numbers of constants. The limit on how many lights you support in a shader is largely a factor of controlling the shader cost rather than one of constant space limits these days. Deferred renderers are very popular these days as well and don't really handle lights in the same way anyway.

DX12 does offer some potential CPU side performance benefits when it comes to updating large numbers of constants efficiently which may well help performance when dealing with lots of dynamic lights but it's not adding any new capabilities beyond what DX11 offers.