At least as I understand it (and there’s a good chance I’m wrong) there’s nothing in US law preventing a state from seceding. It was determined that the way the southern states decided to do it in the runup to the civil war was unconstitutional (and possibly treasonous? seditious? Something like that), but there’s no law saying a state can’t secede. It’s just that there’s no defined process for it and the only way it has been tried was determined to be wrong.

From what I’ve read on the topic, there is technically a way it could be done. The country would basically have to follow the same process as passing a constitutional amendment, just with an additional step.

• The state in question would have to pass a ballot measure to secede
• The state house and Senate would have to ratify that measure with a 2/3rds super majority.
• It would have to be passed as a ballot measure by the majority of the country.
• The US house and senate would have to ratify that measure with a 2/3rds super majority.

So, not technically impossible just so difficult that it is effectively impossible.