Justice Sonia Sotomayor dissented from the Court’s ruling, in so far as she would have preferred to remove the case from the Supreme Court without vacating the lower court rulings against the administration (as is generally, but not always, standard practice when a case is dismissed on mootness grounds).
The Supreme Court has not, so far, dismissed Trump v. Hawaii, the other travel ban case currently before it. But most experts expect that it will ultimately be dismissed as well.
At least one lawsuit has already been filed against the latest travel ban, and more will surely follow, including some by the various state governments, civil rights groups, and others who were plaintiffs in the earlier travel ban litigation.
There is, therefore, a substantial likelihood that this issue will return to the Supreme Court. It might even come back within the next few months, if litigation over Travel Ban 3.0 proceeds at the same rapid pace as that over its predecessors. We have not yet seen the last of this litigation. Not even close.
NOTE: I coauthored an amicus brief in the current Supreme Court travel ban case, on behalf of myself and several other legal scholars, supporting the plaintiffs.