I'm afraid that the status of the US as a signatory of the deal has not changed and since there hasn't been an addendum or something that legally changes the text of the deal to exclude the US, they're still a participant to the deal.
The UNSC is not a court, it is only an executive body that can make decisions in the form of resolutions. Some of these are legally-binding, but that does not mean that the UNSC can confer legal status to states.
The US and other parties to the JCPOA being described as participants is based on facts. The decision of the UNSC to frame Res.2231(2015) in this particular way of addressing the JCPOA participants is not a conferral of legal status, because the UNSC lacks such powers.
U.S. faces tough U.N. battle if it pushes plan to extend Iran arms embargo
“It’s very difficult to present yourself as a compliance watcher of a resolution you decided to pull out of,” said a European diplomat, speaking on condition of anonymity. “Either you’re in or either you’re out.”
However, a State Department legal argument, seen by Reuters late last year, made the case that Washington could still spark a sanctions snapback because it is named as a deal participant in a 2015 U.N. resolution that enshrines the nuclear agreement.
Some U.N. diplomats said that while legal opinions on whether the United States could do this were split, ultimately it would be up to council members to decide whether to accept a U.S. complaint of “significant non-performance” by Iran.
It is a move likely to be challenged, diplomats said.
“It’s going to be messy from a Security Council standpoint because, regardless of what (Britain, Germany and France) think, Russia and China are not going to sign up to that legal interpretation,” said a European official, speaking on condition of anonymity.
https://www.reuters.com/article/us-...lan-to-extend-iran-arms-embargo-idUSKCN22930E