Nothing illegal or unconstitutional about DACA, which is no different from Trump not enforcing the ACA or EPA laws. When Trump emphasizes deporting MS-13 members, he is allocating resources and, by necessity, de-emphazing other deportations. Obama just gave his policy a name; he was transparent.  As recently as Arizona v. United States, the Supreme Court has stated that the Executive branch has prosecutorial discretion in deciding how to allocate resources when enforcing laws. As explained in a Slate article:
“Congress sets only the outer limits of executive power, like a board of directors giving general marching orders to the CEO of a corporation. The president, like the CEO, makes most of the decisions as to policy as well as to enforcement. Congress can prevent the president from doing certain things by refusing to grant him authorization to do them. But Congress generally cannot force the president to enforce a statute if he does not want to."